Sleep science, oriented

Sleep comes in two kinds. Non-REM sleep has three stages: stage 1, a few minutes of handoff from waking; stage 2, real sleep marked by brief bursts of brain activity, where you spend more of the night than anywhere else; and stage 3, deep or slow-wave sleep, named for the slow waves on a brain recording. REM sleep, short for rapid eye movement sleep, looks almost like waking on that recording. Your eyes dart, breathing turns irregular, most vivid dreams happen, and the brainstem holds your arm and leg muscles limp so you do not act the dreams out.

Specific circuits run all of this. Inside the hypothalamus, the suprachiasmatic nucleus (SCN) reads light from the eyes and keeps time, sleep-promoting cells release a calming brain chemical, and the thalamus mutes the senses in most stages before switching back on in REM.

The common slip is treating sleep as one uniform block. Say Lena goes to bed at her usual time but sets the alarm two hours early for a flight. Her deep sleep, concentrated in the first cycles, was mostly done. The cut fell on the REM-rich end of the night, so she lost a large share of her REM. The mix also shifts with age: slow-wave sleep peaks in early childhood and declines through adulthood, so lighter sleep in later life is partly normal.

Rule: judge a short night by which part was cut, because the early hours carry deep sleep and the late hours carry REM.

The two-process model, a framework sleep science has used for four decades, splits sleepiness into two parts. Process S is sleep pressure: it rises while you are awake, steeply at first and then more slowly, and it falls during sleep. Adenosine, a chemical that builds up in the brain while you are awake, is linked to that drive, and caffeine works by blocking it. Process C is the circadian rhythm, the roughly 24-hour clock run by the SCN. Light through your eyes sets it; in the evening it lets melatonin rise, and around sunrise cortisol helps you wake.

The trap is reading sleepiness from hours awake alone. For most people the internal clock is a little longer than 24 hours, so without daily light to pull it back it drifts later, and bright screens late in the evening send it a daytime signal that can hold back melatonin. Picture Omar, awake since 6 a.m., who feels oddly alert at 11 p.m. after an evening on a bright phone. His sleep pressure is high, but his clock has been told it is still day.

Deep sleep shows the pressure being spent. Slow-wave activity is highest at the start of the night, and after sleep loss it grows more intense instead of only lasting longer. That is why you do not have to sleep back every lost hour, though the debt still counts.

Rule: to predict when someone will sleep well, ask both how long they have been awake and what time their body clock thinks it is.

Federal guidance puts adult needs at 7 to 9 hours a night, and adults who sleep under 7 have more health problems. Teens 13 to 18 need 8 to 10 hours, children 6 to 12 need 9 to 12, and the totals climb for younger children, with naps counted until about age 7. Need varies between people of the same age, but the floor matters: the idea that you can train yourself to need less is a myth. You can only get used to feeling tired.

Missed sleep adds up. Losing 2 hours a night leaves a 14-hour debt by the end of a week, and after several nights of losing even an hour or two, people function about as poorly as after a full night without sleep. Sleep loss slows reactions, raises the error rate, makes emotions harder to manage and to read in others, raises the hunger hormone ghrelin while lowering leptin, the fullness hormone, and changes how the body handles blood sugar.

The hardest cost to see is the one inside your own judgment. Short sleepers often rate themselves as fine. Microsleeps, brief lapses into sleep you cannot control, can erase a stretch of a lecture or a drive without your noticing. Sleep-deprived children may look overactive rather than sleepy. And sleeping hours longer on days off is a sign of a weekday shortfall, not a full repayment.

Rule: if you doze the moment you sit still, or sleep far longer on weekends, treat your regular nights as too short.

A survey of more than 147,000 adults in 2009 and 2010 by the Centers for Disease Control and Prevention (CDC) found 4.2 percent saying they had nodded off while driving in the previous 30 days, and self-reports probably undercount it, because people often do not realize they dozed. Reports were higher among people who slept 6 hours or less and among people who snored, a possible sign of obstructive sleep apnea. Commercial drivers, night and long-shift workers, people with untreated sleep disorders and anyone on sedating medicine face higher risk.

You do not have to fall asleep to be impaired. Drowsiness slows reaction time, narrows attention and degrades decisions. Sleep-related crashes cluster at night and in mid-afternoon, and they often involve a single car leaving the road with no sign of braking, because nobody was awake to brake.

The trap is the in-car fix. Say Priya, driving home after a double shift, notices she has missed her exit and drifted onto the rumble strip. She opens the window and turns up the radio, tricks that have not been found to keep drivers awake. Her signs, like frequent yawning and not remembering the last few miles, mean alertness has already dropped. What works is pulling over to rest or handing the wheel to someone rested. Prevention happens earlier, with enough sleep, care for sleep disorders and no alcohol before driving.

Rule: treat a single warning sign as the signal to stop driving, since only rest restores alertness.

Memory consolidation is the process that steadies a new memory and files it into long-term storage. Slow-wave sleep is most tied to declarative memory, meaning facts and events. A leading account holds that new memories are first held in the hippocampus and then, during deep sleep, reactivated and integrated into the neocortex. Three rhythms appear to line up for this: slow oscillations, whose up-states ready neurons for change; sleep spindles, short bursts of fast activity riding those up-states; and hippocampal ripples nested inside the spindles. Skills draw on both REM and deep sleep, so the tidy split of facts in deep sleep and skills in REM is too simple.

Sleep before learning matters as much as sleep after. Without it, focusing and taking in new material get harder, which is why an all-nighter costs you twice.

Naps are the daytime version, with a catch called sleep inertia, the grogginess right after waking. In one controlled study of young adults who had been sleeping short nights, 10-, 30- and 60-minute naps all lifted mood and alertness for hours, but the longer naps brought grogginess that took up to half an hour to clear, and only the 30-minute nap improved memory for pictures learned afterward. One study is a lead, not a law, and naps do not replace a full night. If nights are your problem, a late nap spends the sleep pressure you need at bedtime.

Rule: protect the night after learning, and size any nap to what you have to do right after it.

Dreaming cannot be watched directly, so research works from reports gathered after waking. Early studies found that about 70 percent of people woken from REM recalled a dream, which led researchers to equate the two, but dreams also turn up in non-REM sleep. The continuity hypothesis holds that dreams draw on waking concerns, especially emotional ones, sometimes days later. How often you remember dreams is partly a trait, linked to interest in dreams and imagery ability, so forgetting them says little about sleep quality.

Parasomnias are unusual behaviors or experiences during sleep, and timing is the first clue. Non-REM arousal disorders, such as confusional arousals, sleepwalking and sleep terrors, come from incomplete waking out of deep sleep, so they cluster early in the night and leave little memory. Nightmares and REM sleep behavior disorder come from REM sleep, so they tend to arrive later and are often remembered.

The classic mix-up: Sam's young daughter sits up screaming an hour after bedtime, cannot be comforted, and remembers nothing at breakfast. That is a sleep terror, not a nightmare, and childhood episodes are usually outgrown. An older adult who punches and shouts in step with a dream is a different case, because REM sleep behavior disorder is linked to conditions such as Parkinson's disease. For sleepwalkers, clinicians put safety first: nothing sharp near the bed, and locked windows. Frequent, distressing nightmares are worth raising too, since a therapy that rewrites them while awake has research behind it.

Rule: note when in the night an event happens and whether it is remembered, and bring a new adult pattern to a clinician.

Caffeine keeps you alert by blocking adenosine, the chemical that signals sleep need, so the pressure is still waiting when it wears off. Its effects can last up to 8 hours, which is why an afternoon coffee can show up at bedtime. Sensitivity and clearance vary widely with body weight, medicines and individual differences. Caffeine also hides in places people miss, such as energy bars, gum, supplements and some over-the-counter medicines, and decaf has less caffeine rather than none. The Food and Drug Administration (FDA) lists insomnia and jitters among the signs of too much, and medical experts advise against energy drinks for children and teens.

Melatonin is two different things. Your brain makes it in darkness as a timing signal. The supplement, sold in the United States as a dietary supplement rather than a drug, has a narrower evidence base than its popularity suggests. Reviews suggest it may help with jet lag and, with only a weak recommendation, with delayed sleep-wake phase disorder. Major guidelines found too little strong evidence to recommend it for chronic insomnia, and shift work studies are inconclusive. Labels are unreliable too: one study found 22 of 25 gummy products mislabeled.

Picture Ben, who drinks an energy drink late in the afternoon and then takes melatonin at bedtime to cancel it out. One product is blocking his sleep signal while the other sends a timing signal his clock may not need.

Rule: move caffeine earlier before reaching for a supplement, and check with a health care provider before starting melatonin, especially for a child.

The first habit is arithmetic. Sleep is often the first thing a busy schedule squeezes, and no product makes up for not leaving the hours. After that, consistency does the most work: going to bed and waking at the same times every day, with weekend times within about an hour of weekdays, keeps the clock from drifting.

The hour before bed works best as quiet, low-light time without hard exercise or bright screens, since the light may tell your brain it is still day. Heavy meals within a few hours of bed, nicotine and caffeine all work against sleep. Alcohol is the tricky one: it can help you fall asleep, yet it tends to make sleep lighter and more broken. Federal guidance pairs daylight and daily activity with a quiet, cool, dark bedroom, and keeps adult naps to about 20 minutes, earlier in the afternoon.

Consider Ana, who lies in bed for an hour willing herself to sleep. Trying harder links the bed with being awake. The better move is to get up and do something calm in dim light until she feels sleepy, then go back to bed.

Habits also have limits. Loud snoring with pauses, legs that will not keep still, or months of poor sleep despite solid habits point to causes that habits cannot reach, and blaming yourself for bad habits only delays a visit to a clinician.

Rule: get the basics right first, and if they do not help, look for a cause beyond habits.

Insomnia means difficulty falling asleep, staying asleep or getting sleep of good quality, even when you have the time and setting for it. That condition matters: someone squeezed by an early shift is short on sleep, not necessarily insomniac. Short-term insomnia follows stress or change and passes in days or weeks; chronic insomnia shows up on at least 3 nights a week and lasts longer than 3 months. Worrying about sleep and watching the clock make it worse, because the effort to sleep becomes its own source of alertness.

Federal guidance usually puts cognitive behavioral therapy for insomnia (CBT-I) first for long-term insomnia. It runs about 6 to 8 weeks and combines stimulus control (the bed only for sleep and sex, and out of bed when you cannot sleep), sleep restriction (a limited time in bed that widens as sleep improves), work on anxious thoughts about sleep, relaxation and sleep education. Prescription medicines come in several classes, some meant only for short-term use and some with rare side effects such as walking or driving while asleep, so that decision belongs with a prescriber.

Measuring sleep has its own traps. Polysomnography, an overnight recording of brain waves, eye movements and muscle activity, is the gold standard. Wrist trackers detect sleep well but miss quiet wakefulness, and in one validation study a band's stage-by-stage agreement with the lab was only about 53 percent. So when Jo's tracker reports a great night that she remembers lying awake through, the tracker is the likelier error.

Rule: for chronic insomnia, bring a 1- to 2-week sleep diary and ask about CBT-I before asking about pills.

This chapter is about recognizing patterns, not diagnosing them. Sleep apnea means breathing stops and restarts many times during sleep. In obstructive sleep apnea, the most common type, the upper airway is blocked, often by the tongue and soft palate. In central sleep apnea the brain does not send the signal to breathe. Night signs include frequent loud snoring, gasping and pauses a bed partner notices; day signs include sleepiness, morning headache and a dry mouth. Age, obesity, a large neck, tongue or tonsils, and alcohol, which relaxes the throat muscles, raise the risk. Not everyone who snores has apnea, and only a sleep study settles it. Machines such as CPAP (continuous positive airway pressure) hold the airway open while they are worn.

Restless legs syndrome is an irresistible urge to move the legs, with uncomfortable sensations, that starts during rest, worsens in the evening and eases only while you keep moving. Low iron in the brain and dopamine signaling are implicated, and pregnancy and some medicines, including some antihistamines, can bring it on.

Narcolepsy is a lifelong disorder of sleep-wake control. Its core symptom is daytime sleepiness that more sleep does not fix. People with it often enter REM within about 15 minutes, and REM features leak into waking: cataplexy, a sudden weakness set off by laughter or other strong emotion, plus sleep paralysis and vivid hallucinations at the edges of sleep. Diagnosis uses an overnight sleep study plus a daytime nap test.

Rule: describe the pattern precisely, including what a bed partner sees, and take it to a clinician rather than guessing.

Some sleep problems are timing problems. Delayed sleep-wake phase disorder is among the most common of these: you fall asleep and wake much later than you want, a pattern teens are prone to. The advanced form, more common in older adults, is the reverse. Non-24-hour rhythm disorder lets sleep drift later each day and is common in people who are completely blind, whose clocks cannot read light. Social jet lag, living hours later on weekends, is not a disorder, but it behaves like a weekly trip.

Light is the strongest reset signal. Light after waking shifts sleep earlier; light in the late afternoon or early evening shifts it later. Night-shift workers are therefore advised to block light on the way home, sleep in a darkened room, take a short nap before a shift, use caffeine only early in the shift, and keep days-off sleep overlapping workday sleep instead of flipping back and forth.

Jet lag is the travel version, a temporary mismatch between your clock and local time, most common past three time zones. Because most clocks run a little long, flying east, which needs an earlier clock, is usually harder, and the body adjusts by roughly 1 to 1.5 time zones a day. CDC suggests shifting your bedtime an hour or two toward destination time in the days before you leave, following local sleep times on arrival, seeking daylight, keeping naps to 15 to 20 minutes, and keeping caffeine out of the evening.

Rule: before blaming a sleep problem on willpower, check whether the clock and the schedule point at different times.

For infants, federal Safe to Sleep guidance is to place babies on their backs for every sleep, naps included, until age 1, on a firm, flat, level surface with only a fitted sheet, ideally in the parents' room but not in their bed. The guidance calls back sleeping the most effective step caregivers can take against sudden infant death syndrome (SIDS). Two myths come up often. Side sleeping is not a safe middle ground, because babies roll from side to stomach, the highest-risk position. And back sleepers are not more likely to choke on spit-up, since the windpipe sits above the food pipe.

Teens shift toward later sleep, partly because melatonin is released later and evening light affects them strongly, and they still need 8 to 10 hours. An early school bell on top of a late clock produces chronic school-night sleep loss. Most studies of later start times find teens sleep more, though the change carries real costs for schools.

Older adults still need about 7 to 9 hours, but sleep gets earlier, shorter and lighter, with more waking. Insomnia is the most common complaint after 60, and sleep medicines carry risks that make them a poor long-term plan. Apnea, restless legs and REM sleep behavior disorder become more common, and medicines and pain often play a part. Consider Ruth, 74, who wakes at 5 a.m. and decides she simply needs less sleep now. The need did not shrink; the timing and depth of her sleep changed.

Rule: compare a sleep pattern with what is normal for that age, then look for causes that age alone does not explain.