Part 5: Multiple Choice

Many people believe that catching up on sleep over the weekend cancels out a tired week, but sleep scientists describe a different pattern. When someone goes to bed much later and wakes up much later on Saturday and Sunday than they do on school or work days, their body clock effectively shifts to a new time zone for two days — a pattern researchers sometimes call "social jet lag." The body has to readjust all over again on Monday morning, and that readjustment can leave a person feeling groggy and unfocused, even after what felt like a good long sleep. The size of the shift matters more than most people expect: even a two-hour difference between weekday and weekend wake-up times is enough to produce noticeable social jet lag in some people. Researchers generally recommend keeping wake-up times within about an hour of each other throughout the week, extra weekend sleep included, so the body clock never has to reset itself twice in one week.

Around the start of puberty, something changes in the body's internal clock. The brain begins releasing the sleep hormone melatonin later in the evening than it used to, often by as much as two hours. This shift is not a matter of willpower or bad habits; it is a biological change in the timing of the body clock itself. As a result, many teenagers feel wide awake at ten o'clock at night and only start to feel sleepy well past midnight, even when they know they have to be up early the next morning.

This creates a direct conflict with how most schools are timetabled. If a student's body does not want to fall asleep before midnight, but the alarm still goes off at half past six, the result is a nightly sleep debt that builds up across the week. Some sleep researchers have argued that early school start times work against the biology of the average teenager, rather than around it, and a small number of schools have experimented with starting the day later to see whether it improves attendance and concentration.

Losing sleep does not just make a student tired. During deep sleep, the brain works to turn the day's new information into memories that last — a process often described as consolidation. Skimp on sleep the night after a lesson, and some of what was learned that day may never be properly stored. Sleep researchers now see a full night's sleep less as a reward for a productive day and more as the final, essential step in the learning process itself.

Sleep loss also appears to affect mood. A brain running on too little sleep tends to react more strongly to stress and irritation, and has a harder time calming itself back down. Repeated late nights have been linked to a weaker immune response as well, meaning that students who are chronically short on sleep may catch colds more easily than well-rested classmates.

One of the biggest obstacles to falling asleep earlier is a source of light many teenagers keep beside the bed: the phone screen. The blue-toned light from phones and laptops suppresses melatonin production, so scrolling through a feed right before bed can quite literally delay the point at which the brain is ready to sleep. Sleep researchers do not recommend banning phones altogether, but many suggest dimming screens and switching to low, warm lighting for the hour before bed.

Perhaps the most practical piece of advice, though, has nothing to do with screens. Waking up at roughly the same time every day, weekends included, helps keep the body clock steady. Sleeping in dramatically on a Saturday might feel like a treat, but it can shift the internal clock just enough to make Monday morning feel like a mild form of jet lag.