Sleep and Dreams

Sleep happens in cycles that repeat several times during a single night.

Rapid eye movement sleep is the stage most strongly linked with vivid dreaming.

Deep sleep is the hardest stage to wake from, and it dominates the first part of the night.

Dreams can feel strangely logical while they unfold and confusing once you wake up.

Most dreaming reports describe everyday places and familiar faces rather than exotic settings.

Nightmares are simply dreams with strong fear, and they often wake the sleeper.

Lucid dreaming is the experience of knowing you are dreaming while the dream continues.

The body clock runs on a roughly 24-hour cycle that persists without daylight cues.

Circadian rhythms affect body temperature, hunger and alertness as well as sleep timing.

Morning light helps reset the internal clock, which is why jet lag fades gradually.

Jet lag appears when travel crosses time zones faster than the body clock can adjust.

Yawning is contagious in humans, and seeing someone else yawn often triggers your own.

Scientists still debate why yawning exists, and no single explanation is accepted.

Yawning appears in many mammals and even in some birds and reptiles.

A short nap of around twenty minutes usually ends before the deepest sleep stage begins.

Naps longer than half an hour are more likely to end during deeper sleep.

Many cultures once treated sleep as two separate blocks separated by a wakeful hour.

Before electric lighting, people in many regions slept longer in winter than in summer.

Electric lighting changed bedtime habits by making evening activity possible for many more hours.

People once relied on roosters, church bells and night watchmen to know when to wake.

Beds in ancient Egypt were raised off the floor and sometimes had headrests instead of pillows.

Pillows were once made from stone, wood and straw in different times and places.

Mattresses were traditionally stuffed with straw, feathers or horsehair depending on local materials.

Giraffes sleep for only a few hours a day, often in short standing naps.

Koalas sleep for a large part of the day because their leafy diet provides little energy.

Dolphins rest one half of their brain at a time so they can keep swimming and surfacing.

Sea otters often hold hands while floating so that they do not drift apart in sleep.

Bats sleep upside down, hanging by feet whose tendons lock shut without muscular effort.

Horses can sleep standing up, though they lie down for the deepest stages.

Some seabirds appear to doze while flying, taking brief naps during long migrations over open water.

Bears spend months in winter dens in a state that is lighter than true hibernation.

Hibernation slows the body so much that breathing and heart rate drop noticeably.

Snoring happens when relaxed tissues in the throat vibrate during breathing in sleep.

Sleep talking can occur in any sleep stage and is usually brief and fragmented.

Sleepwalking tends to occur during deep sleep rather than during dreaming stages.

Teeth grinding during sleep is common and often goes unnoticed by the sleeper.

The saying about sleeping tight is often linked to ropes that once supported old bed frames.

Counting sheep as a way to fall asleep appears in writing many centuries old.

Dreams were once widely treated as messages about the future in many ancient societies.

Ancient dream books listed meanings for common symbols such as water, animals and journeys.

Dream interpretation became a publishing business long before modern ideas about the sleeping brain.

A sleep cycle usually lasts about ninety minutes from start to finish.

Newborns sleep for many hours a day, spread across short periods rather than one block.

Teenagers often shift to later bedtimes, a pattern noticed across many countries.

Older adults frequently wake earlier in the morning than they did when younger.

Yawning tends to spread through a room after one person starts, even in silence.

Tiredness can build when sleep is cut short on several nights in a row.

Caffeine blocks signals that make the body feel sleepy, which is why it is so widely used.

Melatonin is a hormone that rises in the evening and helps signal that night has arrived.

Body temperature drops during the night and reaches its lowest point before dawn.

Blind people often report dreams with sound, touch and smell rather than visual scenes.

Most dreams fade quickly after waking, which is why people remember so few of them.