It is said that an adult human needs around 7-9 hours of sleep per day.
Do human beings need around 8 hours of consecutive sleep a day? Is it prejudicial to the body and brain that you don't? Or is that a myth?
There are actually two parts that need to be covered when addressing this question. One part pertains to the question if we even need 8 hours of sleep? Another part, if we need 8 hours of sleep, is there a need for them to be consecutive?
I want to split up this question, because when people talk about different types of sleep patterns, like polyphasic sleep, it is often in an attempt to reduce the total amount one spends sleeping. We want to spend our days being more productive, and sleep is considered an unproductive part because we don't consciously do something during this part of our day. When googling polyphasic sleep this becomes evident with for example some titles referring to "Polyphasic Sleep Cycles Trick Your Body into Needing Less Sleep", "Alternative Sleep Cycles: You Don't Really Need 6-8 Hours!"... I however want to approach the issue of amount of sleep and consecutive sleep separately.
So about the first part... do we need 8 hours of sleep? I have answered this question before , in which I cite a (freely accessible) article of Van Someren . My answer based on this article (along with other studies that point in this direction) is "yes"... or as the title of Van Someren's article states "Doing with less sleep remains a dream".
Studies have shown that when one gets deprived from their sleep need, this can have negative daytime consequences (e.g. in terms of concentration, mood, sleepiness, cognitive performance...) [3-4]. Or as Van Dongen et al. (2003) conclude from their study :
Since chronic restriction of sleep to 6 h or less per night produced cognitive performance deficits equivalent to up to 2 nights of total sleep deprivation, it appears that even relatively moderate sleep restriction can seriously impair waking neurobehavioral functions in healthy adults. Sleepiness ratings suggest that subjects were largely unaware of these increasing cognitive deficits, which may explain why the impact of chronic sleep restriction on waking cognitive functions is often assumed to be benign.
Sleep duration doesn't even have to be reduced in itself to have effects on cognitive function, as a study of Van Der Werf et al. (2009) has shown . In this study an acoustic-perturbation setup was used to reduce the amount of slow-wave sleep (these are the deeper stages of sleep), while keeping total sleep duration intact. When talking about chronic sleep deprivation, e.g. as in insomnia, health outcomes can even be more detrimental, although it becomes harder to disentangle the contribution of sleep in itself, due to higher comorbidities and more health concerns in general at older age.
There are several arguments in favour of why we would need 8 hours of consecutive sleep:
Within the framework of Borbély's two-process model a deviation from the 8 hours of consecutive sleep would result into problems, either because homeostatic sleep pressure is too low, or because the circadian rhythm doesn't promote sleep.
So what about the people that claim they can do on far less sleep?
The BBC article about biphasic sleep has received some attention in the answer by James Riley and some comments a well. For a more elaborate opinion on this article, I refer to . Bottom line of my comment on the article, is that the study cited to support the claim, isn't representative since people were plunged into darkness for 14 hours every day for a month, and that our current society and technological advancement could pose some additional problems in maintaining the biphasic sleep pattern that was claimed to be the prevailing sleep pattern during the 16th century.
Van Someren, E. J. W. Doing with less sleep remains a dream. PNAS 107, 16003-16004 (2010).
Meerlo, P., Sgoifo, A., & Suchecki, D. (2008). Restricted and disrupted sleep: Effects on autonomic function, neuroendocrine stress systems and stress responsivity. Sleep Medicine Reviews, 12(3), 197–210. doi:10.1016/j.smrv.2007.07.007
Goel, N., Rao, H., Durmer, J. S., & Dinges, D. F. (2009). Neurocognitive consequences of sleep deprivation. Seminars In Neurology, 29(4), 320–339. doi:10.1055/s-0029-1237117
Van Dongen, H. P. A., Maislin, G., Mullington, J. M., & Dinges, D. F. (2003). The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation. Sleep, 26(2), 117–129.
Der Werf, Van, Y. D., Altena, E., Schoonheim, M. M., Sanz-Arigita, E. J., Vis, J. C., De Rijke, W., & Van Someren, E. J. W. (2009). Sleep benefits subsequent hippocampal functioning. Nature neuroscience, 12(2), 122–123. doi:10.1038/nn.2253
Borbély, A. A. (1982). A two process model of sleep regulation. Human neurobiology, 1(3), 195–204.
Diekelmann, S., & Born, J. (2010). The memory function of sleep. Nature Reviews Neuroscience, 11(2), 114–126. doi:10.1038/nrn2762
Siegel, J. M. (2009). Sleep viewed as a state of adaptive inactivity. Nature Reviews Neuroscience, 10(10), 747–753. doi:10.1038/nrn2697
I think there is no absolute answer.
edit with reference "The one area about sleep where the experts are in full agreement is that
there is no minimum number of hours that we should all spend sleeping before endangering our health.
Some of us can get by on six hours without feeling any ill-effects and others claim to a minimum of eight. It is a matter of horses for courses." Reference from University of Westminster.
Case in point. My Mother of 95 yrs of age is of sound mind and not had any heart attacks and has lived on 4hrs sleep for decades. Sometimes more but often only this. I too, at 60, have the same pattern. I can work 48hrs straight without ill effects when motivated. I can be awake minutes after a 4hr sleep ready to take on the day.
Getting to sleep is not hard for me either. I can be asleep before the airplane leaves the runway, unless I have good company seated next to me. My body is in great shape and my Doctor agrees.
Like personality, sleep deprivation depends on your environment, your upbringing and your genes.
Polyphasic sleep regimens (which usually are attempted to reduce total sleep time) are difficult to follow, and most who attempt it abandon it after a trial. However, there is evidence that a biphasic pattern was common before the industrial revolution and electric lights. This involved two periods of sleep, both during the night, but with a waking period of one to two hours in between the two phases. More: http://www.bbc.co.uk/news/magazine-16964783