1. You sleep in cycles.
A full sleep cycle takes about 90 to 120 minutes, says psychologist Lisa Medalie, a behavioral sleep specialist at the . You go through four stages, starting with the lightest one and ending with rapid-eye movement (REM). "Usually people wake up for a couple minutes after each complete sleep cycle," she says.
2. You become a cold-blooded animal during REM sleep.
During dream-filled, REM sleep, your body isn't its own furnace. "We lose the ability to thermo-regulate ourselves," says Mark Mahowald, professor of neurology at the . But that's fine because REM periods typically last only 20 minutes or so, though they can be as short as two minutes or as long as 45, says sleep researcher Ursula Voss, a psychology professor at the .
3. You get less REM sleep when you feel uncomfortable.
"When you feel unsafe or cold in your sleeping quarters, you don't enter REM sleep or get as much of it," says Voss. "Your body automatically adjusts so you don't go into that stage. REM is a deeper sleep."
4. Alcohol impacts your REM sleep.
Booze is a snooze-inducing depressant that puts you to sleep, but also lightens your sleep, says pharmacist Keith T. Veltri, clinical pharmacy manager of . You may still remember dreams, though, since the alcohol causes increased arousals — and you can only recall a dream when you wake up during it.
5. Some medicines interfere with your sleep.
Benadryl (or diphenhydramine), a common allergy medicine, may result in shortened REM and fewer dreams, says Veltri. Prescription drugs that can cause nightmares include beta-blockers, which are usually prescribed for high blood pressure, the Parkinson's disease drug, Sinemet, and the smoking-cessation medication, Chantix. Some drugs, such as antidepressants and barbituates, also reduce REM sleep.
6. Babies in the womb spend a lot of time in REM sleep.
Fetuses are almost exclusively in REM, which may be very "important for brain development," says Mahowald. However, they likely aren't dreaming because they have no experience or memories to draw from to create dreams, he adds. Babies spend 50% of their shut-eye time in REM, toddlers 25%, and by the time we're seniors, we are down to about 15% of our sleep in REM.
7. Not all animals experience REM sleep.
Dolphins and whales don't. "It has something to do with their aquatic environment," says Jerry Siegel, professor of psychiatry and director of the at UCLA. Fur seals experience REM and non-REM sleep when they're on land but next to no REM sleep when they're in the water, he says. And an animal's level of intelligence may not necessarily impact REM Sleep. "The platypus has spectacular REM sleep," says Siegel. "This is an animal that has a tiny little brain." If REM sleep were cognitive, he says, "humans wouldn't fall in the middle."