Two of the brain's strongest pulls are the pull toward sleep and the pull toward reward. Both are ancient, both are run by deep brain systems we rarely notice, and both can be hijacked, by coffee and screens in one case and by drugs in the other.

The architecture of sleep

For a long time sleep was thought of as the brain simply switching off. In 1953 Eugene Aserinsky and Nathaniel Kleitman at the University of Chicago, watching sleepers wired to EEG machines, reported periods of rapid eye movement during sleep, now called REM sleep. During REM the brain is nearly as active as when awake, most vivid dreams occur, and the muscles of the body are temporarily paralyzed, which keeps us from acting out our dreams.

The rest of the night is non-REM sleep, which ranges from light dozing to deep, slow-wave sleep marked by large, slow EEG waves. The brain cycles through these stages about every 90 minutes. Deep sleep dominates the early cycles, and REM periods grow longer toward morning. Research suggests that sleep helps fix the day's learning into lasting memory, and work in mice since 2013 suggests that sleep also speeds the flushing of waste from brain tissue, though how much this applies to humans is still debated.

Two forces

Whether you feel sleepy depends on two forces. The first is a body clock. The master circadian clock is a small cluster of cells in the hypothalamus called the suprachiasmatic nucleus. It keeps a rhythm of roughly 24 hours and is reset each day by light detected by special cells in the eye. In darkness it signals the pineal gland to release melatonin, a hormone that helps prepare the body for sleep.

The second is sleep pressure, which builds the longer we are awake. One cause is a chemical called adenosine, which accumulates in the brain during waking hours. Caffeine works by blocking receptors for adenosine, hiding the tiredness signal without removing it, which is why the fatigue returns when the coffee wears off. The cost of ignoring these forces is real. A 2000 study by Ann Williamson and Anne-Marie Feyer found that after 17 to 19 hours without sleep, performance on some tests was like having a blood alcohol level near 0.05 percent, and longer stretches were worse still.

The reward system

In 1954 James Olds and Peter Milner at McGill University placed electrodes in rats' brains and let the rats deliver tiny electric pulses to themselves. Rats would press a lever again and again to stimulate certain parts of the brain, sometimes thousands of times an hour. The researchers had found part of a reward circuit. At its center is a pathway from the midbrain to the nucleus accumbens, a region beneath the front of the brain, which releases dopamine when something good is happening or about to happen.

Addictive drugs act on this circuit, directly or indirectly. Cocaine blocks dopamine's reuptake; opioids such as heroin and fentanyl release the brakes on dopamine cells while also acting on the body's own pain system. With repeated use comes tolerance, the need for more of the drug to get the same effect, and withdrawal, unpleasant symptoms when it stops, as the brain adjusts to the drug's constant presence.

Disease or learning?

The US National Institute on Drug Abuse has long described addiction as a chronic brain disease, pointing to lasting changes in reward and self-control circuits. Defenders say this framing reduces blame and opens the door to medical treatment. Critics, including the neuroscientist Marc Lewis, argue that every strong habit changes the brain and that addiction is better seen as deeply ingrained learning, shaped by circumstance. A famous study supports the role of circumstance. In the early 1970s the sociologist Lee Robins followed American soldiers returning from Vietnam, where many had used heroin. Most of the soldiers who had been addicted in Vietnam stopped using heroin after returning home; in her reports only about 5 percent were addicted again within a year. A new setting, away from the cues and stresses of war, changed outcomes dramatically. Both views agree that addiction is real, harmful and treatable.

Opioid overdoses kill by acting on the brainstem, where they slow and finally stop breathing. Naloxone reverses an overdose by blocking opioid receptors and pushing the drug off them, an antagonist of the kind described earlier in this course.

Worked example: the all-nighter

A student plans to stay up all night before an exam, fueled by coffee, and to review notes until dawn. What does sleep science predict?

Caffeine will block adenosine receptors and keep her alert for a while, but the sleep pressure is still building. By morning she will have been awake around 24 hours, impaired roughly as much as being legally drunk in many countries, and her circadian clock will be pushing hardest for sleep in the hours before dawn. Worse, she will skip the deep sleep and REM that help store what she studied, so material crammed late may not stick. Driving to the exam is the most dangerous part of her plan. A better plan is to study earlier, sleep a full night, and let the brain do some of the consolidation work for her.