We've all been there: It's been a long week at work, so Friday night, you reward yourself by going to bed early and sleeping in. But when you wake up the next morning (or afternoon), light scathes your eyes, and your limbs feel like they're filled with sand. Your brain is still lying down and you even have faint headache. If too little sleep is a problem, then why is extra sleep a terrible solution?

Oversleeping feels so much like a hangover that scientists call it sleep drunkenness. But, unlike the brute force neurological damage caused by alcohol, your misguided attempt to stock up on rest makes you feel sluggish by confusing the part of your brain that controls your body's daily cycle.

Your internal rhythms are set by your circadian pacemaker, a group of cells clustered in the hypothalamus, a primitive little part of the brain that also controls hunger, thirst, and sweat. Primarily triggered by light signals from your eye, the pacemaker figures out when it's morning and sends out chemical messages keeping the rest of the cells in your body on the same clock.

Scientists believe that the pacemaker evolved to tell the cells in our bodies how to regulate their energy on a daily basis. When you sleep too much, you're throwing off that biological clock, and it starts telling the cells a different story than what they're actually experiencing, inducing a sense of fatigue. You might be crawling out of bed at 11am, but your cells started using their energy cycle at seven. This is similar to how jet lag works.

But oversleep isn't just going to ruin your Saturday hike. If you're oversleeping on the regular, you could be putting yourself at risk for diabetes, heart disease, and obesity. Harvard's massive Nurses Health Study found that people who slept 9 to 11 hours a night developed memory problems and were more likely to develop heart disease than people who slept a solid eight. (Undersleepers are at an even bigger risk). Other studies have linked oversleep to diabetes, obesity, and even early death.

Oversleep doesn't just happen as a misguided attempt at rewarding yourself. The Harvard Nurses Study estimated that chronic oversleep affects about 4 percent of the population. These are generally people who work odd hours, have an uncomfortable sleep situation, or a sleeping disorder.

People who work early morning or overnight shifts might be oversleeping to compensate for waking up before the sun rises or going to sleep when it's light out. Doctors recommend using dark curtains and artificial lights to straighten things out rather than medication or supplements. Apps like the University of Michigan's Entrain can also help people reset their circadian clock by logging the amount and type of light they get throughout the day.

When you go to bed, your body cycles between different sleep stages. Your muscles, bones, and other tissues do their repair work during deep sleep, before you enter REM. However, if your bed or bedroom is uncomfortable—too hot or cold, messy, or lumpy—your body will spend more time in light, superficial sleep. Craving rest, you'll sleep longer.

If everything's just fine with your sleep zone but you still can't get under the eight hour mark, you might need to go see a doctor. It could be a symptom of narcolepsy, which makes it hard for your body to regulate fatigue and makes you sleep in more. Sleep apnea is a potentially more serious disorder where you stop breathing while you slumber. It's typically caused by an obstructed airway, which leads to snoring. However, in a small number of sufferers, the brain simply stops telling the muscles to breathe, starving the brain and eventually forcing a gasping response. In addition to all the other terrifying aspects of this disease, it's not doing your quality of sleep any favors.

No surprise, drugs and alcohol might also be causing you to sleep too much, as does being depressed (In fact, oversleep can contribute to even more depression). But no matter what's causing it, too much sleep is not good for your long term health. Rather than kicking the can down the road, try getting some equilibrium between your weekend and weekday sleep.