The Sleep Drive

What’s Behind Those Feelings of Sleepiness

We try to ignore it. We try to fight it off and drive it away. Sleepiness, we tell ourselves, is just a feeling. Something more of the mind than the body. Something we should be able to control with the exercise of a little willpower.

Ah, if only it were that easy. The truth is, those feelings of sleepiness run a lot deeper than you may think. They are your body’s way of trying to tell you something — something you can’t afford to ignore.

Your body is trying to tell you to get more sleep!

Most sleep scientists today believe that the human body has a specific daily requirement for sleep and that it’s more or less a fixed part of our physical make up. Everyone has their own daily requirement — some people need more, others less — but for most of us, it’s pretty much the standard eight hours a day, give or take an hour. In other words, we need approximately one hour of sleep for every two hours spent awake. And your body wants very much to maintain this 2 to 1 ratio.

In order to do this, your body generates those well-known feelings of sleepiness. It uses these feelings to prod, cajole, even coerce you into getting the right amount of sleep. You can resist it up to a point. But it’s a battle you’re destined to lose. Eventually, sleepiness will intensify into drowsiness and you’ll be worn down bit by bit, until sleep overtakes you at last, pulling the plug on your consciousness in order to get its due. Like a heartless utility company that hasn’t received payment, it’ll shut off your lights!

The idea that you feel more tired when you get less sleep is not exactly the scientific revelation of the century. What fascinates us here is the mechanism your body uses to bring this about. Scientists call it the “homeostatic sleep drive.” It is an innate drive — an urge — for sleep that slowly builds with every waking hour and has its origin in complex neurochemical interactions taking place deep inside the brain. Why is it called “homeostatic,” you ask? A quick explanation is in order.

Homeostasis Explained

The human body has this really nifty way of regulating itself called homeostasis. The term refers to the tendency of any complex system or organism to keep itself at an internal constant, even when everything around it is in flux. And so it is with you: For every external or environmental change your body encounters, it needs to make a compensating internal adjustment to keep things running smooth and steady. Just about everything inside of you, from your body temperature to the regulation of your blood pressure, is controlled homeostatically.

You can think of the homeostatic control in your body as working a bit like the thermostatic control in your house. You’ve got your thermostat set to automatically maintain a certain indoor temperature. So if it gets hot outside, and the temperature inside your house starts to climb, the thermostat responds by cranking up the A/C until things cool back down. If it gets cold outside, and the inside temperature falls below the preset temp, it turns on the furnace to warm things up.

This isn’t much different from the way your body uses homeostasis to keep itself at a normal temperature of 98.6°, whether you’re in the middle of a blazing 110° summer afternoon or an arctic 20-below-zero winter morning. On a hot day, your body automatically compensates for the heat by turning on its own A/C — cooling you down by increasing perspiration, respiration, and blood flow to the skin. When you’re in a cold environment, your body turns up the furnace — by burning more calories, constricting blood vessels, making you shiver to keep warm, or giving you goose bumps.

Everything’s Under Control

Homeostasis is also how your body maintains a consistent blood pressure (responding to increases in BP by decreasing heart rate), blood sugar levels (by secreting insulin or glucagon from the pancreas), the concentration of CO₂ in the blood (reducing excessive concentrations by increasing respiration rates), as well as your body’s level of hydration (removing excess water through the kidneys). It would not be too much of an exaggeration to say that good health and homeostatic balance are one and the same thing.

Fortunately, most of this physiological fine-tuning is done completely on autopilot. But there are some things your body can’t do without a little help from you. In some cases, your body must stimulate you into consciously engaging in certain behaviors or activities that support homeostatic balance. It does this by giving you a variety of urges, motivations, appetites and impulses.

For instance: Your body maintains an optimal level of hydration by making you feel the sensation of thirst when you need more fluids, so you’re motivated to drink water. And it regulates your caloric intake by making you feel hungry when you need food, motivating you to eat. The more your body needs something, the more intense the motivation, until it becomes a kind of primal, unstoppable urge.

And this is exactly how your body’s homeostatic function drives you to get the right amount of sleep — by making you feel sleepy. Think of sleepiness as basically a hunger for sleep.

The Sleep Homeostat

Remember our thermostat analogy? Just as your house has a thermostat to control temperature, your body is said to have a “sleep homeostat” to control sleep. Let’s say your sleep homeostat is set for eight hours of sleep a day. If you miss a couple of hours one night, the sleep homeostat will respond by turning on the sensation of sleepiness, to motivate you to get more sleep.

If you miss four hours, six hours, or God forbid, a whole night’s sleep, the sensation of sleepiness will be that much more intense, until eventually you get to that bombed-out, stick-a-fork-in-me-I’m-done feeling of total exhaustion.

The body’s sleep homeostat does this simply by keeping a running tab of how much time you spend awake. The people who study this stuff call this your “sleep load.” Every minute you’re awake, from the instant you roll out of bed in the morning, you’re accumulating a sleep load. Conversely, you get rid of your sleep load by (you guessed it) sleeping.

So for every two minutes you’re awake, you’ll eventually need to compensate with one minute of sleep — thus after a 16 hour day, you’ll need about 8 hours of shuteye, depending on your personal sleep need. After a solid night’s sleep, your sleep load is back down to zero; but once you wake up, it starts accumulating all over again. And if you’re getting anything less than your daily requirement, you’ll actually wake up with that much of a sleep load already pressing down on you.

Anytime you’re missing sleep, the inevitable result is going to be daytime sleepiness.

Deep Inside the Brain

Scientists don’t yet fully understand exactly how the sleep drive works at the physiological level, but a neurotransmitter called adenosine seems to play a key role here. Adenosine progressively accumulates in the brain tissue during your waking hours and dissipates while you sleep. It has an inhibitory effect on the brain cells that are responsible for producing alertness — which is to say, it makes you feel sleepy. With every hour you’re awake, you build up more and more of the stuff. So the longer you go without sleep, the more tired it makes you feel.

It is an accident of nature — and a fortunate thing for us — that the caffeine molecule just happens to be shaped like an adenosine molecule. This allows molecules of caffeine to fit nicely into the adenosine receptors inside the brain. The result is that caffeine temporarily blocks adenosine from reaching the brain and inducing the feeling of sleepiness. Now you know why that cup of coffee (or that can of Diet Coke®) is such an indispensable part of getting through those long hours at work.

But while caffeine may be wonderful stuff, it won’t keep you going forever. It can mask the symptoms of sleepiness but it can’t free you from your biological need for sleep. There’s just no substitute for the restorative powers of a good, solid eight hours a night. We’re at our best when we find the right balance between sleep and wakefulness, rest and activity. And the body’s homeostatic sleep drive is designed to help us achieve this by using the feeling of sleepiness to signal us when we’re falling short.

So the next time you find yourself getting sleepy in the middle of the day, don’t try to ignore it, don’t chalk it up to laziness. Listen to the simple message your body is trying to give you: You need to get better sleep.