Most animals sleep in several bouts rather than one block, and humans are treated as the exception. Our single consolidated night is either a species trait or a product of the short dark period that electric light leaves us. Those predict different things. So normal individuals were moved from a conventional sixteen-hour to an experimental ten-hour one, and their sleep was recorded.
The night came apart in the middle. Sleep episodes expanded and usually divided into two symmetrical bouts, several hours each, separated by a waking interval of one to three hours. Human sleep had turned . The duration of expanded too, as did the nocturnal phase of rising sleepiness measured under a constant routine. So the internal timing shifted, not just the behavior.
Sleep in other animals is typically unconsolidated and polyphasic, taken in several bouts across the day and night. Human sleep is one long block, and that difference is usually treated as a fact about our species. The alternative is that it is a fact about our lighting.
A photoperiod experiment can distinguish them. Normal individuals were transferred from a conventional sixteen-hour photoperiod, roughly what modern life supplies, to an experimental ten-hour one, roughly a long winter night without electric light.
Their sleep episodes expanded and usually divided into two symmetrical bouts of several hours each, with a waking interval of one to three hours between them. That middle-of-the-night waking is not insomnia in this setting. It is what the sleep did when the dark period was long enough to allow it.
The physiological measures moved with the behavior. The duration of nocturnal melatonin secretion expanded, and so did the nocturnal phase of rising sleepiness under a . That indicates the internal processes controlling sleep and melatonin had been modified by the change in photoperiod. Previous work suggested these results could be simulated with dual oscillators entrained separately to dawn and dusk, or with a two-process model having a lowered sleep-onset threshold during the dark period.
wat. Given ten hours of darkness instead of sixteen hours of light, sleep split into two long bouts with a wakeful gap between them.
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It reframes the consolidated night as a product of our light environment rather than a human given. Waking for an hour in the middle of a long night is what the physiology does when given the chance.
How do we know?
Wehr transferred normal individuals from a conventional sixteen-hour photoperiod to an experimental ten-hour photoperiod, recording sleep episodes alongside nocturnal melatonin secretion and sleepiness under a constant routine protocol.
What is still unknown
A small photoperiod experiment from 1992, and the conclusion rests on a laboratory light schedule rather than on how anybody lives. It describes what sleep can do, not what is wrong with a single block.
Learn something?
Where to next
What is the difference between jet lag and tiredness from a long flight?sleep
What keeps time in a body that has no clock in it?sleep
How fast does a child’s body clock shift without electric light?melatonin