Lights out.

The number is not in dispute. The Centers for Disease Control and Prevention recommends seven or more hours of sleep a night for adults between 18 and 60, and the American Academy of Sleep Medicine and the Sleep Research Society reached the same figure in a joint consensus statement in 2015. The behavior is not in dispute either. In a CDC analysis of a 2014 national survey, 35 percent of American adults said they usually slept less than seven hours. Most people know the rule and break it anyway, and the reason is worth understanding. Sleep is the one appetite that cannot be commanded. Nobody can decide to fall asleep. What a person can decide is almost everything else, and that is where the work is.

A study published more than twenty years ago shows what a short night actually costs. In 2003, Hans Van Dongen, David Dinges and colleagues at the University of Pennsylvania published the results of an experiment in the journal Sleep. They kept 48 healthy adults, aged 21 to 38, on four, six or eight hours in bed a night for fourteen days, and compared them with a group kept awake for three nights straight. By the end, the people restricted to six hours a night performed on cognitive tests about as poorly as people who had gone up to two full nights without sleep.

The worse finding concerned how they felt. Their ratings of their own sleepiness rose at first and then flattened, while their performance kept falling. The authors wrote that the subjects seemed largely unaware of their growing deficits. That is the trap of the short night. The gauge people use to decide whether they are getting enough sleep is the same gauge that sleep loss quietly breaks.

Because sleep cannot be ordered, the useful controls sit around it. The first is the alarm. The CDC’s list of habits begins with a simple instruction, which is to go to bed and get up at the same time every day, and of those two times the wake-up is the one you can actually enforce. Pick it, keep it seven days a week within about half an hour, and count back seven and a half hours to find the latest reasonable bedtime. If the alarm is set for 6:30, the lights go out by 11:00. Holding the morning steady gives the body clock a fixed point, and the evening tiredness tends to follow.

The second control is the room. The CDC’s advice for the bedroom fits in a phrase: quiet, relaxing and cool. Put blackout curtains or a blind on the window and cover every standby light. Turn the thermostat down at night. Then take the work out, including the laptop, the mail and the television. The CDC suggests turning off electronic devices at least thirty minutes before bed, and a plain alarm clock lets the phone spend the night charging in the kitchen, which is the simplest way to keep that promise.

The third control is what goes into the body in the hours before bed. In a 2013 study in the Journal of Clinical Sleep Medicine, Christopher Drake and colleagues at Henry Ford Hospital and Wayne State University gave twelve healthy sleepers 400 milligrams of caffeine, roughly two to three cups of coffee, at bedtime, three hours before it and six hours before it. Even the dose taken six hours early cut measured sleep by more than an hour, and the volunteers mostly did not notice. The authors suggested avoiding caffeine after 5 p.m. For someone with an 11 o’clock bedtime, the study itself argues for an earlier line. The afternoon cup you do not feel is still in the room at midnight.

Alcohol works the other way. It can make a person drowsy at bedtime and then break up sleep later in the night, so the drink that seemed to help is often behind the 3 a.m. wake-up. A large, late dinner does similar damage, and the CDC advises against both before bed. Regular exercise helps, and the federal physical activity guidelines list better sleep among its short-term benefits. None of these rules is dramatic. Taken together, they move the odds of a full night from luck to arrangement.

Everyone eventually meets the lying-awake loop. You go to bed on time, the mind replays the day, and a glance at the clock makes the replay louder. Sleep medicine’s answer is almost too plain to believe. If you have not fallen asleep after about twenty minutes, get up, go to another room and do something dull in low light, such as reading a paper book. Return only when you feel sleepy, and keep the wake-up time the next morning regardless. Turn the clock to face the wall.

This is more than folk advice. In 2016, the American College of Physicians recommended cognitive behavioral therapy for insomnia, a structured program built on techniques like this one, as the initial treatment for adults with chronic insomnia. If the loop keeps returning for weeks, that is a reason to see a doctor rather than read another article about it. The same goes for anyone who is told they snore loudly or seem to stop breathing in the night, because those are signs a professional should assess.

The case for all of this is not self-improvement. It is that sleep is the only part of the day in which a person gives up control completely, and the only way to make that surrender go well is to prepare for it. Keep one wake-up time, one cool dark room and one early coffee cutoff, and protect those three for a month before changing anything else. The night will still be out of your hands. Everything leading up to it will not.