The science of sleep

Yawning is a reflex triggered primarily during periods of state change, such as following transitions of sleeping and waking. Yawning may help promote alertness.

Yawning is contagious in in social species - humans, chimpanzees, lions, and dogs, and shows variability in expression, with some more prone to contagious yawning than others. Yawning as a result of observing someone else yawn does not develop until after infancy. 

Contagious yawning may have evolved to synchonise and coordinate group behaviour, providing survival benefits to members of the group. It also may have evolved to increase vigilance within a group. Seeing a group member yawn as an indicator that they are experiencing diminished arousal, may increase the observer's vigilance, to compensate for the lowered vigilance of the yawner.

It was once believed that sleep was unique to creatures with a brain. Now we know that creatures like worms, jellyfish, and hydra experience a sleep-like state. Most animals, even the simplest ones, experience a restful state.

Some animals, like koalas, sleep for up to 22 hours a day.

Cats sleep for about 15 hours a day, usually taking multiple catnaps, in addition to short periods of deeper sleep, both during the day and night. Dogs on the other hand, sleep at night when people do, but take several daytime naps to supplement their nighttime sleep.

Worker ants sleep almost five hours a day in the form of 250 one-minute power naps. 

Dolphins sleep an average of eight hours per day. As they rest, they turn off half of their brain - one hemisphere sleeps, while the other remains alert to the need to return to the surface to take breaths and look out for predators. Every two hours, the hemispheres switch, so the other half rests.

Because sleep is all about alternating between stages, the overall restorative effect cannot yet be replicated.

Pills usually tackle just one stage, such as deep sleep, without dealing with the other stage. We don't know how the whole process of sleep works yet to be able to replicate it. 

Getting extra sleep —  more than the recommended 7 hours — is important for recovering from sleep deprivation or from a medical condition. Children need more sleep for optimal development. While there is evidence for increased risk of chronic disease with persistent short sleep duration, there is no evidence that long sleep is related to health and mortality. 

Experiemental research shows decreased performance on cognitive tasks following two consecutive nights' sleep. Performance returns to pre-deprivation levels with sufficient sleep recovery. Evidence from cognitive and performance measures show that sleep recovery following brief sleep deprivation periods can return to baseline levels. 

Research has found that compared to polysomnography tests, used in clinic to diagnose sleep disorders, sleep trackers are accurate 78% of the time when identifying sleep versus wakefulness. When estimating time taken to fall asleep, accuracy falls to around 38%.

Sleep trackers estimate sleep patterns by measuring body movement and heart rate. Polysomnography tests track brain waves, heart rate, breathing, blood oxygen levels, and body and eye movements during sleep. Analysing brain wave patterns is the only definitive way of measuring sleep versus wakefulness and tarcking sleep stages.