Most of the questions below refer to the article: The cruel myth of polyphasic sleep

Polyphasic sleep will not improve your cognitive performance
The best way to minimize time spent sleeping is to free run your sleep
Taking more than one nap per day quarrels with productivity goals
Adults cannot emulate baby sleeping habits
You cannot improve your mental output by sleeping less
Your family doctor will not instantly spot the damage resulting from polyphasic sleep
Polyphasic sleep looks different before you begin your trial
It is a myth that polyphasic sleep gives you more REM sleep
Caffeine levels won't affect instability of polyphasic sleep pattern
Your polyphasic sleep experiment is doomed to fail
Vegan diet will not significantly change your sleeping habits
Polyphasic Sleep article is biased

Polyphasic sleep will not improve your cognitive performance
(anonymous, Jul 19, 2002)
Question:
I read about polyphasic sleep. Apparently it is excruciatingly hard to get it started, but once people did, they got higher average scores on cognition and recall tests. It makes sense. It's how animals sleep. Just watch a cat or dog. It's how babies sleep. I heard it is how Buckminster Fuller slept
Answer:
False! You read only one thing right: polyphasic sleep is excruciatingly hard. It is not only painful to start. It is even more painful to sustain when your hopes of gaining a few extra hours per day lay down in ruins as you keep on your struggle with the natural sleep mechanism. Buck Fuller, Edison, Tesla and others might have had very strange sleep habits, but they certainly did not go far against their natural sleep needs. If they tried, we might still be a few years back with industrial development. You cannot follow animal sleep. Each animal has its own sleep pattern. You do not want to follow a nocturnal rat. Nor will you be able to sleep with one hemisphere on like a dolphin. Or sleep in a standing position or in flight (like some birds). The oscillator changes also as we age, hence we convert from polyphasic babies, to biphasic or nearly monophasic adults. Laboratory experiments consistently indicate that normal humans are unable to consistently sustain polyphasic sleep. What is worse, their cognitive performance in polyphasic sleep experiments is not better. It is dismal. Dr Claudio Stampi, who studies polyphasic sleep regimes, recommends polyphasic sleep for those who need to maximize alertness for a set condition of sleep deprivation (e.g. as in solo sailing). This cannot be confused with maximizing alertness overall. The formula for maximum overall alertness is: free running sleep. Most people are biphasic or even monophasic when they free run their sleep. Anything against the free running rhythm is bound to result in mental torture and cognitive deficit

The best way to minimize time spent sleeping is to free run your sleep
(Virgil, Monday, June 24, 2002 8:58 AM)
Question:
I've come across many evangelists for the polyphasic sleeping schedule (Uberman sleep schedule) as adopted by Edison, Da Vinci, etc. I've seen various modifications of both of their sleeping schedules: 

1. Da Vinci method, briefly, required the person to operate on a 4-hour basis. At the end of every 4-hour wake phase, the person sleeps for 15-30 minutes 
2. The modified version is called CyberSleep. The person sleeps between a half-hour to one-hour. The method also requires an extended sleep of 2 hours during the early morning period 
3. Minors and Waterhouse (1981) proposed a method based on an 8-hour total sleep time. The anchor methods requires sleep to be "anchored" to an unvarying 4-hours of sleep every night at the same time, but allows the remaining 4-hours to be taken at any other time during the day, as the person desires. The "flexible" 4-hour portion must be used as a whole and not fragmented into component naps. 

Are these sort of esoteric and unconventional sleeping schedules good ideas to emulate or not? I would love to cut down on the amount I sleep
Answer:
Polyphasic sleep is "recommended" only to those who want to contribute to the understanding of sleep at the cost of their health and intellectual performance. All forms of sleep enforced with an alarm clock will increase your overall demand for sleep. This means that: if you use an alarm clock, either (1) you will sleep longer or (2) you will feel more miserable. Artificial sleep schedules will dramatically reduce your mental capacity. A healthy individual in normal conditions will find it difficult to fall asleep 4 hours after the main sleep episode unless that episode was unnaturally cut with an alarm clock resulting in sleep deprivation. Polyphasic schedules are very appealing in theory, and many people try them out just to give up within a week or a month (depending on the ability to suffer through the mental misery). Those who try to adjust to any unnatural schedule will suffer an unspeakable torment of the mind. Polyphasic sleepers regulate their sleep with an alarm clock until they reach the breaking point. Human self-experimenting guinea pigs collapse into a sound life-saving 5-8 hour sleep towards the breaking point and then resume the polyphasic schedule with a sense of guilt. That sense of guilt is calmed with exculpatory terminology such as "weekend break", "re-energizer", "bonus sleep", etc. Polyphasic experimenters may happen to sleep less but their intellectual performance will be dramatically undercut. Some polyphasic sleep theories are based on the false premise that the body can adapt to any sleeping rhythm. Other researchers try to find a natural polyphasic rhythm that would minimize the pain of sleeping little. Scientists have studied the so-called phase response curve of the circadian rhythm, where the impact of various sleep affecting factors is shown to move the sleeping schedule forward or backward. The obvious conclusion is that we can rather painlessly move the major circadian low little by little in a desired direction. However, a healthy normal individual will not be able to chop the rhythm into a desired number of pieces. Monophasic sleep or biphasic sleep are the norm in healthy individuals. Biphasic sleep is rarely composed of two major sleep episodes. Usually it has a form of a major episode (nocturnal sleep) and a minor episode (siesta). What great inventors might have experienced is an irregular sleeping schedule in conditions of hypomanic excitement. In such conditions, sleep may become fragmented, but it will still not submit to a designer schedule. Great catnappers nap when they feel they need to. Often, they can accurately predict when and how much they will need to nap. If you want to minimize time spent sleeping and maximize your learning results: free run your sleep. Get rid of the alarm clock!

It is a myth that polyphasic sleep gives you more REM sleep
(Hermann Klinke, Feb 03, 2006, 16:17:52)
Question:
I think that you are mostly right with what you wrote in the article on polyphasic sleep. But there is one thing that you misunderstand: Ubermen try to eliminate all sleep phases but the REM phase, because that is the most important phase.
Answer:
As there is no "Uberman science", one can only address a collective view of polyphasic bloggers, which is a mix of rumor, pseudo-science and misunderstood science. It is true that some bloggers claim that the goal of polyphasic sleep is to get REM-only sleep. As this goals is entirely implausible, the article addresses a more plausible variant in which the sleeper attempts to compress sleep stages through sleep deprivation and consequently gain waking time. The roots of the REM-only misunderstanding can probably be tracked back to old sleep models that have been rubber-stamped by mainstream sleep research.

Historically, the importance of REM sleep for memory and learning was documented before we became truly aware of the role of slow-wave sleep. Consequently, articles and books on sleep are peppered with an overemphasis on the role of REM sleep in learning as compared with SWS. Over time, REM deprivation studies received lots of criticism. Today, we know that the natural harmonious interplay of uninterrupted NREM and REM sleep is essential for memory, learning and creativity.  

Cruel sleep deprivation studies actually show that sleep deprived rats can live longer if REM deprived than if NREM deprived. Similarly, napping human subjects reported that it is Stage 4 NREM that feels most restorative.

In sleep deprivation induced by polyphasic schedules, REM sleep will occur faster due to sleep stage compression. Yet it is the slow-wave sleep that is the primary target of homeostatic upregulation strongly determined by the duration of prior waking. As REM sleep is far more associated with the circadian phase, its proportion in sleep will actually drop, esp. in naps initiated in the subjective evening period. You may want to study sleep models by Alexander A. Borbély and Peter Achermann, which nicely explain the mechanics of these processes. Laboratory findings seem to indicate that the drop in REM gradually recovers towards the baseline over successive days of sleep deprivation, but the reversal is never complete. In other words, you will get less REM sleep on a polyphasic schedule as compared with a free running schedule. This REM sleep diet is as much absolute (as measured in minutes) as it is relative (when compared with Stage 4 NREM). Getting more REM in polyphasic sleep is a widespread myth.

The problem of REM deprivation becomes more pronounced if you use an alarm clock when waking up from naps. By using the alarm clock, you statistically hit REM sleep more often as its proportion always increases over sleep time. This is why polyphasic sleepers often remember their dreams on awakening. That's not a sign you get more REM. It's a sign you are destroying your REM sleep. By using very short blocks of sleep, you affect REM even further by a strong homeostatic upregulation of Stage 4 NREM that displaces whatever REM you can get.

You probably know that alarm clocks are bad for sleep and for health in general. They also interrupt the natural memory consolidation and optimization cycles of sleep. This is explained in non-biological language in: Polyphasic sleep for dummies.

If you (1) do not fight sleepiness and (2) wake up from your naps naturally, the problem of sleep disruption does not occur. However, it is impossible to regularly fit a pre-planned polyphasic schedule without some help from alarm clocks. This comes from the fact that the only stable sleep patterns in healthy individuals are mono- or biphasic. Polyphasic sleep patterns may stable and sustainable in various cases of hypersomnia. When the sleep control system is disrupted and the homeostatic sleep component works in overdrive, frequent napping may occur and be recommended (e.g. narcolepsy, infection, medication, etc.). Also a degree of sleep block fragmentation may also occur as a result of stress, social life, excitement, staying up late, etc. Those disturbances may occasionally allow for days with more than one nap occurring naturally. If you give up the alarm clock, you take away the major culprit that makes polyphasic sleep unhealthy. However, without an alarm clock, it is your body that will decide the sleep schedule, not your pre-planned "rationalized" schedule graph.  

If your goals is to get many naps with lots of REM, you might want to know that more than two naps with solid REM sleep are diagnostic for narcolepsy - a disorder that can turn the life of sufferers into a misery.

Some reading on the subject of the link between sleep stages and learning:

• Articles by Robert Stickgold, on the importance of all stages of sleep, should be studied at primary levels to ensure no kid ever contemplates experimenting with his (girls are more sensible here) sleep patterns. Here is a recent one from Nature available on-line: http://www.nature.com/nature/journal/v437/n7063/full/nature04286.html
• For more on the role of NREM in memory see: Campbell, I.G., Guinan, M.J., and Horowitz, J.M.. Sleep deprivation impairs long-term potentiation in rat hippocampal slices. J. Neurophysiol. 88, 1073–1076 (2002): http://jn.physiology.org/cgi/content/full/88/2/1073 
• And a scary reading for sleep deprived Ubermen: Guzman-Marin, R., Suntsova, N., Stewart, D.R., Gong, H., Szymusiak, R., and McGinty, D. Sleep deprivation reduces proliferation of cells in the dentate gyrus of the hippocampus in rats. J. Physiol. 549, 563–571 (2003):
  http://jp.physoc.org/cgi/content/full/549/2/563 

It's a pity that polyphasic sleepers, instead of studying the subject, prefer to dig "wisdom" from fellow bloggers and engage in a stultifying game of gossip that does not advance our understanding of sleep hygiene

Adults cannot emulate baby sleeping habits
(Alex Gordia, Feb 05, 2006)
Question:
Why all this rant against polyphasic sleep? Don't babies sleep polyphasically? There is even a saying about good sleep: "to sleep like a baby"
Answer:
First of all, babies sleep for far many more hours than the alleged polyphasic sleepers (say, 16 hours instead of 3). Secondly, the polyphasic sleep pattern is lost gradually as soon as in the first year of life. Try as you might, you cannot possibly keep sleeping polyphasically, nor for 16 hours, unless you are seriously sleep deprived. Thirdly, babies do not use alarm clocks to shred their sleep to pieces. Last but not least, polyphasic sleepers (despite a widely circulated myth), lose REM sleep in the first order. Babies, on the other hand, may get as much as 60% REM, without which their cerebral cortex would not even develop correctly (as evidenced in sleep deprived kittens). The origin of the rant comes from the potential damage that may result from further dissemination of the myths surrounding polyphasic sleep

Caffeine levels won't affect instability of polyphasic sleep pattern
(Aigars Mahinovs, Feb 06, 2006, 11:21:01)
Question:
Could avoiding caffeine beverages help in adaptation to polyphasic sleep?
Answer:
Because of a relatively slow elimination of caffeine and its impact on adenosine receptors canceling homeostatic sleepiness, ingesting caffeine later than 5-7 hours before a nap will make taking a nap more difficult (except cases when ingestion takes place directly before a nap).

It is true then that avoiding caffeine shall make taking multiple naps somewhat easier. Yet it won't remedy the problem of grogginess when waking up in  subjective night. The problem with polyphasic sleep is the asymmetry of the circadian cycle (which is only marginally affected by caffeine), and a slow build up of homeostatic sleepiness. Even complete abstention from caffeine will not generate sufficient homeostatic sleepiness without a degree of sleep deprivation. Reversely, taking drugs that would activate adenosine receptors would result in sleep patterns that would rather resemble narcolepsy than polyphasic sleep. That would go precisely against the goal of polyphasic sleepers, which is to sleep less. Polyphasic sleep pattern is inherently unstable, and changing levels of caffeine will have no bearing on this fact whatsoever. 

As for normal healthy sleep (which polyphasic sleep is not), abstention from coffee is not necessary, but all caffeine drinks should be optimally taken only within the first hour after awakening

Vegan diet will not significantly change your sleeping habits
(Aigars Mahinovs, Feb 06, 2006, 11:21:01)
Question:
Can vegan diet help entrain polyphasic sleep? It is well known that herbivores sleep less
Answer:
The short answer is: No. Vegan diet will not help you sleep polyphasically. The long answer is:

• polyphasic sleep cannot be naturally entrained for circadian reasons, and no diet can change this fact
• instead of minimizing sleep, you should optimize the amount of sleep. This means you should get exactly as much sleep as is needed to fulfill its neural functions
• the simple rule of the thumb for designing diets for optimum sleep is: focus on maximizing the general health effects of the diet. Healthy body will ensure healthy sleep
• vegan diet is sustainable, but requires a great deal of nutritional knowledge to be safe in the long-term. Vegetarian diets are far easier to compose for an average man, esp. with some backup from sea foods. Probably the safest and the simplest diet for optimum health is a diet high on plant foods, with some meat added as a safety valve. The optimum amount of meat is probably impossible to determine due to the sea of contradictory research findings. Another good rule of the thumb is: listen to your body. If modifications to your diet bring bad results then you might be heading in the wrong direction. But your body can fool you too, and you need to use solid, well-established nutritional guidelines to make sure you do not stray into untested waters. Knowledge of evolution and past human eating habits is helpful too
• as for the actual impact of the vegetarian diet on the demand for sleep, you might want to address this question to a nutritionist specializing in studying vegetarian diet as the field is huge, complex, and full of contradictory findings compounded by vested interested of those who promote vegetarianism on religious or ethical ground and feel little remorse while stretching the truth on the connection between the diet and health. Interestingly, there has been quite a deal of research on how deficient vegetarian diet (e.g. short on carnitine or exogenous FAs) might worsen hypersomnia or fibromyalgia
• from the point of view of neurophysiology, variations in healthy diet are unlikely to have a major impact on your sleep. You may only want to watch caffeine, alcohol, exotic herbal products, toxins, and all substances with a substantial effect on the nervous system. Otherwise, the diet should have only a minor impact on the demand for sleep, circadian patterns, homeostatic sleepiness (with a major exception for caffeine), progression of sleep stages (with a major exception for alcohol), or neural efficiency of sleeping. The reason for this is the same as in many other cases of homeostasis. The organism looks for specific nutrients in foods while striving at retaining homeostatic balance throughout all systems. Rare foods, herbal preparations, pharmacological intervention, etc. can always change or unbalance internal equilibria, but a standard healthy diet is far less likely to do so. It takes an extraordinary nutritional error to stop the human heart. It is even harder to stop the gene-based body clock.
• it is not true that herbivores sleep less, as there are many exceptions to the rule (there are herbivores that sleep three times as much as short-sleep carnivores). There is however a correlation, which says that the decrease in sleep time is faster with the increase in weight in herbivores than it is in carnivores. In other words, heavy herbivores, like giraffe, indeed sleep very little. This correlation may be explained by changes in metabolism, but it could also reflect a different lifestyle. A predator may eat once and then spend many hours on digestion, an elephant keeps munching all day long to sustain its energetic needs, while a gazelle needs to maximize vigilance to ward off an attack from a long sleeping lion
• the correlation between diet type (herbivore vs. carnivore) and the length of sleep links sleeping habits with eating habits of a species, not eating habits of an individual. While humans are omnivorous, you won't become a herbivore, and allegedly a short sleeper, by enforcing new eating habits

Your family doctor will not instantly spot the damage resulting from polyphasic sleep
(H., Feb 03, 2006, 16:17:52)
Question:
I know bloggers who adapted successfully to polyphasic sleep. One is also doing regular checkups with his doctor to make sure polyphasic sleep is not harmful. No damage has been found.
Answer:
Most of bloggers who claim success with polyphasic sleep seem to have trimmed their standards of satisfactory alertness and creativity. When statements such as "my successful experiment" and "groggy" come together, you can be certain that "the experiment" does not effectively maximize their alertness and productivity. There might be also many other explanations; however, none of these is the disappearance of the natural circadian rhythmicity that makes polyphasic sleep impossible.  

As for visiting your family doctor, his ability to detect trouble on polyphasic regiment is not much different from his ability to see trouble in a novice smoker. The damage is not done instantly and it is not obvious. Moreover, a big part of the damage is the opportunity cost. It is not only what you have done to your health, but also what you have not accomplished as an individual due to your reduced mental capacity. Even less so, can your GP detect long-term effects of possible damage at the neural level and neural function to your long-term growth and intellectual accomplishment. Things you do not learn today may change the entire course of your life. No one can estimate that cost. Even substantial neural damage in Alzheimer's disease is not easily diagnosed, and it does not become obvious until you are in advanced stages of cells loss. 

Visiting your GP for a checkup is always a good idea. However, it is pretty useless in preventing damage done by polyphasic sleep. 

Taking more than one nap per day quarrels with productivity goals
(Placebo, Feb 09, 2006, 04:57:04)
Question:
I am experimenting with polyphasic sleep (blog). You claim that having more naps adds no benefit. This I believe to be patently wrong, as adding extra naps is what eased my difficult stretches.
Answer:
You are right that in polyphasic sleep, every extra successful nap will be preciously helpful in restoring your mental energy. Even in a normal sleeper who is not sleep deprived, an additional nap is likely to bring increased alertness and improve mental performance. What is not true is that we should strive at having many naps during our working day. If your goal is maximum productivity, and if you sleep well in the night (i.e. you are not sleep deprived), then any nap attempt at times other than the siesta time will be wasteful. More often than not, you will not even manage to fall asleep. Resting with your eyes closed does not yield a fraction of benefit of an actual successful nap. Moreover, even if successful, an extra nap forced in in the morning is likely to interfere with your afternoon nap. Similarly, an evening nap may result in shortening the night sleep. Those extra naps may bring incremental improvement in performance, but will reduce the overall efficiency of sleep. Our biphasic nature makes it quite clear, we should take a single nap in the afternoon. For some people, even this will be too much, and monophasic pattern is their optimum

Your polyphasic sleep experiment is doomed to fail
(Placebo, polyphasic sleep blog, South Africa, Feb 09, 2006, 12:35:49)
Question:
If you could convince me somehow that everybody fails at it and I cannot be any different then I might give up [my polyphasic sleep experiment]. One thing I've learnt about science is that theories can and often are wrong. I respect science, and part of science involves attacking a theory. I can do this by testing it. It's about reward and risk. At worst it takes me some time to correct my sleep patterns again. The reward is far greater than the risk in my opinion, and leads me to be a human guinea pig :) I've already shown that polyphasic can be successful for a short period of time. But it is by no means plain sailing. Now I would like to see if I can turn that around.
Answer:
Your willingness to change the dogma is praiseworthy. The willingness to become a human guinea pig led to numerous discoveries in the past. Yet you should always look for areas where you stand a reasonable chance of success. If a number of areas of science "conspire" to tell you that your effort is doomed, if a number of researchers in controlled conditions produced discouraging results, if you have evidence for the whole concept to be a product of human love for myth-making, if your background equips your poorly for the job (knowledge of sleep physiology would dramatically improve your chances), you if you know your plan is injurious to your health, your choice may only be explained by insufficient study of the subject. With the right attitude, your potential discoveries will not be undermined if you devote a few months of intense study to this issue. Without some prior investigation, we might all live in a dreamland of efforts directed exclusively at telekinesis, telepathy, teleportation or levitation. The world of "dreamland science" would stagnate. Nobody would work on mundane aspects of food science or material chemistry. We do need crazy researchers ready to waste their lives for a dream. Those people produce paradigm shifts and they will teleport objects one day. Some great discoverers avoided priming their brains with current research in fear of becoming biased. Yet statistically, knowledge is beneficial. And the status quo, makes it easy to predict that your efforts will bring no fruit. 

Please use SleepChart to track your progress. It would be highly beneficial for your experiment if others could see and comment upon your SleepChart data and your homeostatic sleep graphs (note that circadian graphs are useless in polyphasic sleep due to the fact that you regularly interrupt sleep with an alarm clock). 

Link to your site will be published at supermemo.com (Feb 15, 2006).

Polyphasic Sleep article is biased
(Hong Sio, July 25, 2006, 16:30:24)
Question:
From your article on polyphasic sleep I would say you are somewhat biased. Perhaps what drives this view most strongly is the humor section, where you isolate bloggers' comments. It may serve to ridicule, but not for someone who is searching for empirical evidence which is quite scarce. The "for dummies" section did not help that impression one bit. Perhaps another approach toward your argument would be to agree with polyphasic sleepers that those naps go straight to REM sleep. And from that point, provide evidence as to how non-REM sleep is important to the growth and maintenance of the body
Answer:
The article is as biased against polyphasic sleep as you would expect an article written by an oncologist to be biased against smoking. 

The article emphasizes the fact that extracts from blogs are added for purely comedic reasons. The simple reasoning that demonstrates that polyphasic sleep cannot be entrained is presented much earlier in the article (starting with "To sleep or not to sleep polyphasically"). You do not need "for dummies" nor "excerpt" sections to understand "the evidence" you are looking for. The evidence is not exactly "empirical" as there have been very few attempts to study polyphasic sleep in a lab (sleep researchers are rather interested in "serious" matters, not "teenage fads"). Yet a few simple facts of chronobiology will help you understand that the concept "refreshing polyphasic sleep" is just an urban myth that spreads like wildfire in sleep-deprived and phase-shift maladapted community of young students and programmers.

The articles explains that "going straight to REM" is a myth (see also this FAQ). As for the importance of NREM for learning, you will find a plethora scientific and popular scientific articles on the net (at supermemo.com, you can have a look at Good sleep, good learning)

Polyphasic sleep looks different before you begin your trial
(kuactet, USA Educational, Wednesday, August 16, 2006 2:10 PM)
Question:
Having read your article about polyphasic sleep, I am totally unconvinced. The only thing that all of the people who failed at polyphasic sleep have in common is this: they did not follow the procedure. They either a) fell asleep when they shouldn't have, b) overslept, or c) did something completely unexpected and irrational. For example, I quote, "I thought... I'd try by starting to gradually redistributing my sleep to 6 periods throughout the day". If there is a procedure which works, you should follow the procedure instead of going ahead and doing your own idiotic thing because you think it might be better. The procedure is go cold turkey, and follow the sleep schedule. If you want to demonstrate people that have failed at polyphasic sleep, you have to show me people that have followed the procedure and failed, not random people that failed at whatever they wanted to do instead of polyphasic sleep.
Answer:
The argument against polyphasic sleep is entirely biological. Knowledge of chronobiology tells us polyphasic sleep is impossible. Personal stories included in the article were added for comedic effect only. They are obviously no proof one way or the other. 

Perversely, one could say that there is another thing all polyphasic beginners have in common. Before their trial, they all subscribe to your reasoning above. Once they read wild claims of the power of polyphasic sleep and blogs with (untrue) claims of success, they build a model of the adjustable body clock. Depending on the sources, beginners will choose a polyphasic schedule that is said to have been working for someone or allegedly for everyone. Needless to say, no such sleep schedule can ever succeed. The next step is the transition attempt. Again, depending on the sources, a polyphasic adept may believe going cold turkey is better, or gradual adaptation is better. Whichever the path, the outcome is inevitable: sleep deprivation, problems with oversleep, problems with the adherence to the schedule. No wonder, all those who tried polyphasic sleep will inevitably look for errors in their approach and look for possible modifications to the schedule. At that point they become cannon fodder for those who are at the beginning of the road. After all, for the believers in the adjustable body clock, those struggling individuals may appear weak, and their experimentation, as you say "idiotic". 

In other words, you can either study the biological argument against polyphasic sleep and give up, or you can plunge into your own time-wasting experiment and soon begin your own experimentation that others will call "idiotic". In the end, the outcome is preordained

You cannot improve your mental output by sleeping less (#12668)
(David C., Dec 24, 2006, 10:10:54)
Question:
I generally agree that you should not try to sleep less, but even you will agree that sometimes less sleep is better (i.e. wake-up at the end of a cycle versus wake-up later (more sleep), but in the middle of 90 min REM-NREM cycle)
Answer:
It might be better to have one sleep interrupted at the shallowest sleep stage as opposed to a deeper sleep later on. However, it does not affect the simplest sleep optimization strategy: sleep as much as your body needs.

In normal healthy conditions, you will always naturally wake up at the end of the last natural cycle of sleep. Healthy conditions include absence of stress, correct sleep hours, quiet and comfortable sleeping environment, etc.