1.5 Sleep

Consciousness is your awareness of yourself and your surroundings, and it comes in levels. Being wide awake, daydreaming, drowsy and deeply asleep are all different states of consciousness.

Your sleep-wake cycle follows a circadian rhythm, a biological clock that runs about 24 hours. Light is its main cue: darkness triggers the release of melatonin, which makes you sleepy, and morning light suppresses it. Jet lag happens when you cross time zones faster than your clock can adjust. Shift work, especially overnight shifts, forces people to sleep when their clock says 'be awake,' which can harm sleep quality, mood and health. Teenagers' clocks tend to shift later, which is one reason early school start times are hard.

Researchers identify sleep stages by their EEG patterns. When you're awake and alert, brain waves are fast and irregular; when you're relaxed with your eyes closed, they slow into alpha waves.

• NREM-1: the light transition into sleep. You may feel hypnagogic sensations, like the feeling of falling or a sudden jerk, or images that seem real.
• NREM-2: you're clearly asleep but still fairly easy to wake. The EEG shows bursts called sleep spindles. Adults spend more total time in this stage than any other.
• NREM-3: deep, slow-wave sleep with large, slow delta waves. It's hardest to wake someone from this stage, and the body does much of its repair here.
• REM sleep: rapid eye movement sleep. Brain waves look almost like being awake, yet the body's major muscles are essentially paralyzed. That's why REM is called paradoxical sleep. Most vivid dreams happen here.

One full cycle through the stages takes about 90 minutes, and you repeat it four to six times a night. Early in the night, cycles contain a lot of NREM-3 deep sleep. As the night goes on, deep sleep shrinks and REM periods grow longer, so most of your REM and dreaming happens in the early morning hours.

If you're deprived of REM sleep, your brain makes up for it the next time you sleep by entering REM sooner and staying longer. This is called REM rebound, and it suggests REM serves an important purpose.

Two leading theories explain sleep. Restoration theory says sleep lets the body and brain recover, repairing tissue and clearing waste built up during the day. Memory consolidation theory says sleep helps organize and strengthen the day's memories, moving them into long-term storage. Students who sleep after studying generally remember more than those who stay up all night.

Two theories explain dreams. Activation-synthesis theory says that during REM the brainstem sends out random neural activity and the cortex tries to weave it into a story, so dreams may have no hidden meaning. Consolidation theory says dreams reflect the brain processing and storing the day's experiences and memories. You won't be tested on Freud's psychoanalytic theory of dreams.

Poor sleep hurts attention, memory, mood, reaction time and immune function. Treating sleep disorders and keeping a regular schedule improves daytime performance and well-being.

• Insomnia: ongoing trouble falling or staying asleep.
• Narcolepsy: sudden, uncontrollable attacks of sleep during the day, sometimes dropping straight into REM.
• REM sleep behavior disorder: the normal muscle paralysis of REM fails, so people physically act out their dreams and may injure themselves or a partner.
• Sleep apnea: breathing repeatedly stops during sleep, briefly waking the person many times a night, often with loud snoring and heavy daytime sleepiness.
• Somnambulism (sleepwalking): walking or doing other actions during deep NREM-3 sleep, more common in children.