Birds, fish, reptiles and other mammals have at least one thing in common with us: they sleep. Sleep is a central part of our lives and is clearly crucial. Although sleep has fascinated philosophers, writers and scientists for centuries, it wasn’t until the early 1950s that scientific research began in earnest. Since then, sleep science has revealed much about the structure and patterns of sleep. Even so, its origins and functions remain largely mysterious
What is sleep?
Strictly speaking, the term “sleep” only applies to animals with complex nervous systems. Nevertheless it is possible to identify sleep-like states in invertebrates that allow us define sleep more broadly. These include cycles of rest and activity, a stereotypical body position, lack of responsiveness and compensatory rest after sleep deprivation. Insects in particular have a state very similar to sleep, as do scorpions and some crustaceans.
“Insects in particular have a state very similar to sleep, as do scorpions and some crustaceans”
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Even microorganisms, which lack a nervous system, have daily cycles of activity and inactivity driven by internal body clocks known as circadian clocks. The origins of sleep might therefore date back to the dawn of life 4 billion years ago, when microorganisms changed their behaviour in response to night and day.
Some researchers consider sleep part of a continuum of inactive states found throughout the animal kingdom. Once we understand exactly what aspects of an organism benefit from these states, we may be able to provide a meaningful answer to the question of whether simple organisms sleep.
Reasons for rest
There are many explanations for sleep, ranging from keeping us out of harm’s way to saving energy, regulating emotions, processing information and consolidating memory. Each has strengths – and weaknesses too. Rather than seek a single, universal function of sleep we might do better to study its influence at each level of biological organisation.
At the level of the whole organism a primary function of sleep may be the regulation of autonomic nervous activity such as heart rate; sleep disorders are often associated with dysfunction of the autonomic nervous system, such as an abnormal heartbeat. At the level of the brain it may support memory consolidation by reducing the amount of information travelling through the central nervous system. However, memory consolidation occurs when we are awake too.
At the level of nerve cells, sleep alters firing rates of neurons and also changes the temporal distribution and synchronisation of firing across networks of cells, which may alter their connectivity. The regulation of nerve-cell connectivity, called synaptic homeostasis, can help prevent the nervous system from becoming overloaded. Support for this idea has come from recent studies of fruit flies (Science, vol 332, and ).
One neglected role of sleep in humans is social isolation. As social animals, we may need sleep to consolidate the rules and insights of our complex social lives.
Half awake
Sleep feels like an on-or-off condition, but brains can be awake and asleep at the same time. This phenomenon is well known in dolphins and seals, animals that can sleep “uni-hemispherically”: one half of their brain is asleep while the other half shows electrical activity characteristic of wakefulness.
A study in rats published last year found that after prolonged wakefulness some neurons go offline and display sleep-like activity (). Tellingly, this mosaic brain state is accompanied by occasional lapses in attention. Sleep researchers are investigating if human and other animal sleep is a “global” state or whether the process of sleep can, to some extent, be regulated locally. There is mounting evidence for the latter. For example, the most active brain regions during wakefulness subsequently undergo deeper sleep for longer ().
This localised view of sleep could lead to a better understanding of cases when wakefulness intrudes into sleep, such as in sleep-talking, sleepwalking and episodes of insomnia in which people report being awake all night even though recording brainwaves (see “Slumber cycles”) from a single location suggests they have been asleep.
It also promises to explain how sleep can intrude into wakefulness, such as during lapses of attention when we are sleep-deprived. These “micro sleeps” can be particularly dangerous when driving and various ways to detect them have been developed, for instance by monitoring how a car moves relative to white lines on roads or analysing the movements of the eyes for signs of sleepiness ().
Slumber cycles
During sleep, complex changes occur in the brain. These can be observed with an electroencephalogram (EEG), which measures the brain’s electrical activity and associated brainwaves.
After lying awake for 10 minutes or so we enter non-rapid eye movement sleep or NREM sleep. NREM sleep is divided into three stages, NREM1, NREM2 and NREM3, based on subtle differences in EEG patterns. Each stage is considered progressively “deeper”.
After cycling through the NREM stages we enter rapid-eye-movement or REM sleep. The EEG during REM sleep is similar to wakefulness or drowsiness. It is during this stage that many of our dreams occur.
Each cycle lasts for about 1.5 hours and a night’s sleep usually consists of five or six cycles.
In addition to changes in brain activity, sleep is also characterised by a reduction in heart rate of about 10 beats per minute, a 1 to 1.5 °C fall in core body temperature as well as a reduction in movement and sensation.
See graphic: “Sleep scientists break sleep into four distinct stages”.