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What Makes You Fall Asleep?

Recharging the Batteries and Measuring Sleep (1:23)

Daniel Cohen, MD, MMSc, formerly of Harvard, explains how we can recharge our internal battery to avoid sleep debt. He also explains how scientists measure sleep needs.

True or false? You can tell how sleepy you are.

That’s false!

Being sleepy makes it hard to know just how tired you are. When you are sleepy, you make more mistakes than when well-rested and may nod off in an instant.

Most people think they consciously decide when to go to sleep. Not true. Sleep is so important to survival that the brain orders us to go to sleep. We ignore this command at our peril.

Scientists call the pressure to sleep the sleep drive. Your brain is telling you, "You need to sleep." If you tune in to this message, you’ll turn off your TV or computer, and wind down other activities. You will go to bed. You’ll likely fall asleep in a few minutes. 

Sleeping seven to nine hours turns off your sleep drive, recharges your inner battery, and lets you to start the next day refreshed. From the minute you wake up, your inner battery starts running down. 

At about the fifteenth hour of wakefulness, your inner battery is depleted. Sleep-promoting chemicals build to a critical level. You will start to feel drowsy...very, very drowsy. Your thinking will dull. Your eyelids may droop. You may nod off briefly, and then jerk back to wakefulness. These brief lapses are known as microsleeps. They may occur repeatedly. 

If you stay awake 16 hours or longer, you probably will find it harder and harder to think straight. You may become cranky, impulsive, and clumsy. 

After being awake for 20 hours or more, people perform a variety of mental and physical tasks as poorly as people with a blood alcohol concentration of 0.08 percent. That’s the legal criterion for alcohol intoxication in the United States. As with those who have had too much to drink, people who are sleep deprived also lose the ability to recognize how impaired they are.

After 20 hours or more of wakefulness, people often fall asleep involuntarily. They may experience more microsleeps, though still may not realize they are having such lapses. A lot can go wrong in just a few seconds. Have you ever wondered, "What was the last signal I passed"? 

Indeed, microsleeps lasting only a short time may have dire consequences. In just a few seconds, a train or car going 55 miles per hour will travel the length of a football field.

People who work long hours, particularly when finishing work at night, might find that the most dangerous time of day is the hour or so after leaving work. They may be driving alone at highway speed in the dark. It takes only two seconds to veer across a lane into oncoming traffic, or to run off the road.