Sleep changes as people grow, and dreams, animals and astronauts all have their own sleep stories. Sleep is about quality as well as hours. Good sleep is uninterrupted and refreshing. The body also builds a sleep drive, a desire to sleep that grows stronger the longer a person has been awake.
How much sleep a person needs depends on age, and it differs from person to person. As of November 2026, babies under four months old often sleep between 14 and 17 hours a day. As of November 2026, infants aged 4 to 12 months need about 12 to 16 hours of sleep, naps included. Toddlers aged 1 to 2 need about 11 to 14 hours, and preschoolers aged 3 to 5 need about 10 to 13 hours, naps included, as of November 2026.
In toddlers, napping appears to be necessary for memory consolidation, executive attention and motor skill development. Babies spend more time in REM sleep than older children and adults, and their sleep cycles are shorter. As kids get older they spend less time in REM sleep and their sleep cycles get longer.
Teenagers' body clocks shift later, so they feel sleepy later at night and want to wake up later in the morning. An adult's natural internal clock runs on average 24.2 hours, and daily light cues reset it to 24 hours. Travel across time zones, called jet lag, and shift work change the normal sleep-wake cycle.
Sleep comes in two types: non-REM sleep, which goes from light to deep, and REM sleep, when most dreaming happens. Brain waves measured during sleep show clear patterns for each sleep stage. Dreaming is most common and intense during REM sleep, but it can happen in any sleep stage. Dreams during REM sleep are typically more vivid, fantastical or bizarre.
On average, most people dream for around two hours each night. Some people dream in color while others dream in black and white. Dreams are involuntary: a person does not choose them. They can also be forgotten in the blink of an eye.
In a lucid dream the dreamer is actively aware that they are dreaming. Recurring dreams repeat the same imagery across many dreams over time. During REM sleep most muscles are paralyzed in a condition known as atonia.
Many kinds of animals, not only people, have REM periods during sleep. Elephants sleep very little: some researchers have recorded a total of just 2 hours a day. Little brown bats sleep about 20 hours a day. Horses, like giraffes and elephants, do not sleep much, and they can sleep standing up. Birds also sleep with one side of the brain asleep while the other stays awake, and only the eye linked to the sleeping side is closed.
Then there is space. As of November 2026, the International Space Station orbits Earth every 90 minutes, so crew members see 16 sunrises each day. The frequent change from darkness to light makes it hard for the body to keep a natural circadian rhythm. Astronauts sleep strapped into a sleeping bag in a small sleep station, held by restraints so they do not float around.
Words to know
- toddlers: very young kids
- lucid: aware you dream
- atonia: muscles switched off
- circadian: daily body rhythm
- recurring: coming back again
