Eight hours of sleep, still sleepy in class?

The full narration of the video.

One viewer asks: why do I sometimes sleep a full eight hours and still nod off in class? Yet on less than six, I feel fine? Most of us think sleepiness is just about how many hours you got. That's only half right. Three things are really behind it: the moment you wake, the sleep you owe, and the classroom itself.

First, the moment you wake. Sleep doesn't stay at one depth. It's like an elevator: down deep, then back up. Each ride takes about ninety minutes, four or five rides a night. The alarm yanks you out of sleep. Your head is wrapped in fog that usually takes ten minutes or more to clear. Scientists call this sleep inertia. If you were already short on sleep, or your body still thinks it's the middle of the night, the fog gets thicker.

So take the same eight hours. Sleep till noon on the weekend and your body clock slides later. When Monday's alarm rings, your body is still in the middle of the night. It's like jet lag. Next, the sleep you owe. Why does six hours feel fine? There was an experiment in the US: healthy young adults slept just six hours a night for two weeks straight.

Two weeks later, they took reaction tests. They missed about as many as people who'd stayed up all night. But their own sleepiness ratings rose a little in the first few days, then barely moved. Short on sleep for long enough, you go numb to your own tiredness. Not feeling sleepy doesn't mean you've slept enough.

Third, the classroom itself. Besides the middle of the night, your body clock has a small dip around one or two in the afternoon, lunch or no lunch. Add sitting still, just listening, nothing keeping you up, and the sleepiness you've been hiding slips out. One more number: experts recommend eight to ten hours a night for thirteen- to eighteen-year-olds. For a teenager, eight hours is just the starting line.

So it's not the extra two hours that make you sleepy. It's when you wake, the debt you've piled up, and that afternoon dip. The advice researchers give most often: get up at about the same time every day, and don't sleep till noon on weekends. So tell me: which class is your sleepiest? The first one, or the one right after lunch?