What Is Monophasic Sleep & Why Is It the Most Common Sleep Cycle?
6 min read
Last Modified 26 May 2023 First Added 1 October 2021
At first glance, most of us would think that sleeping in one block of time, at night, is the norm. But were things always this way? Sleep cycles are a little more complicated than that. Looking across the globe and through the aeons of history, we can quickly see a range of different approaches to sleep. Most of us nowadays sleep in a monophasic sleep cycle. Siesta culture does still exist in some countries (although it is fading) and there’s an argument that those of who enjoy a nap in the day, regardless of how frequently, are actually sleeping biphasically. Here, we explore monophasic sleep cycles: what they are and whether they’re the natural sleep cycle or if the reason most of us sleep in one block is a response to modern lifestyles.
A monophasic sleep cycle consists of a single instance of sleep during any given 24-hour period. It’s the most common type of sleep for modern humans as we tend to take our sleep in one single period in any given 24 hours.
While there’s evidence that biphasic sleep was the standard in pre-industrial eras, there are relatively few modern cultures where biphasic or polyphasic sleep (sleep periods of several sessions) is common – and pressures from global capitalism have seen the ‘siesta’ threatened even in countries where this practice has been historically common.
Looking into the etymology of the phrase, we can see how mono comes from the Greek monos for alone, or in this context, one. The phasic part of the term has a longer etymological history. It originates from the Greek phainein which means to show. This then evolved into phasis meaning appearance. Its current form – phase – is what we call a loaned word. It’s actually a French word that has become part of the English lexicon.
The pervasiveness of sleep across species indicates that it’s an evolved trait that goes back many hundreds of millions of years and while being unconscious for extended periods of time seems counterintuitive from a survival perspective (any period of unconsciousness leaves an animal vulnerable), sleeping serves a wide variety of theoretical and proven evolutionary benefits.
Having gained empirical evidence as neuroscience has developed, the restorative nature of sleep is theorised to be a primary reason for its evolution. While animals deprived of sleep will lose immune function and die in a matter of weeks, sleep is one of the few occasions that things such as protein synthesis, tissue repair, muscle growth and more will occur.
Sleep, thanks to numerous studies of the brain using PET scans and other recently developed technologies, has been proven to play an important role in the structure and organisation of the brain. In addition, sleep and dreaming are theorised to play a part in the organisation of information, in planning and in processing.
While in much of western society resources are no longer scarce, that was not the case for most of human or, for that matter, mammalian history. As such, the need to reduce energy requirements and expenditure is theoretically one evolutionary reason for sleep.
While there’s no definitive answer to the question of why we sleep as yet, its presence in the majority of species, the negative consequences of sleep deprivation and the many positive aspects of quality sleep are enough to indicate its usefulness.
The question remains, however, why do the majority of people sleep in one long period? In truth, this was not the case for the majority of human history. In fact, there’s a wealth of historical documentation to support the idea that sleep was actually taken in two periods during the night with a period of wakefulness between. This means that the natural sleep pattern for humans is likely biphasic, though with both periods during a single night.
The likely reason for this is another evolutionary advantage. Humans evolved as a diurnal (active during the day) species; we don’t see especially well during the night, and the hunting method that helped early humans catch large prey, which saw groups track them to exhaustion, works best if you can see where you’re going. Additionally, with fewer large predator species active during the night hours, long periods of inactivity and inattentiveness were far safer in the dark. While we can theorise the reasons for nocturnal sleep, the reasons for early humans to sleep in two phases is even less well known – though it would be useful, one supposes, to have some members of a group alert during the night – what we can say is that the reason for single or monophasic sleep is almost certainly the combination of three factors: industrialisation, electric light and the formalisation of the working day.
Industrialisation moved large sections of humanity out of agricultural jobs and indoors, electric light allowed for the extension of waking and working hours and it became common for workers to perform their jobs for more than half of any 24-hour period. As such, a single period of sleep became the most economic use of time and quickly became the norm (outside of equatorial regions where, due to high temperatures during the day, another form of bi-phasic sleep continued – the afternoon nap).
There’s no ‘one-size-fits-all’ answer to this question any more than there is to the ‘how much sleep do you need?’ The majority of people will operate best with either a single period of sleep, or with one short and one long period in any 24 hours, but there are other sleep cycles that have been studied. Meta analysis of these studies, however, do point to a drop in the performance of certain core physical and mental processes with polyphasic sleep, but there are always outliers. The easy conclusion to draw is that monophasic and biphasic are likely to be the best cycles to guarantee peak cognitive and physical performance, but with sleep – as with so much else – the best will vary from person to person.