Right up there with functions like breathing and blinking, sleeping is an activity you totally take for granted. If you’re like most people, you go about your daily routine. You go to work, go to lunch, go home, pick up the kids from soccer, work out—and eventually pass out.

And little do you know, your body is doing a major overhaul and recharge every time you catch some zzz’s. It affects every system of the body, your emotions and mood, how well you think and how well you perform.

“In general, sleep is critical for survival,” says Michael Smith, PhD, Director of the Behavioral Sleep Medicine Program at Johns Hopkins University and a sleep insights consultant for Fitbit. “Almost every living thing has some form of rest, and almost every bodily process benefits from sleep.”

Interestingly, scientists still aren’t exactly sure why humans have adapted to sleep. There are theories, like early research that shows sleep might be the body’s way of eradicating a buildup of waste from the brain. When you hit the sack, the space between brain cells is flooded with cerebrospinal fluid. This seems to “clean” the brain, “sweeping away the detritus of the day,” Smith explains.

There’s also research linking sleep loss and sleep disorders to a myriad of health issues, which offer clues as to why this basic activity is so essential. The link to heart disease suggests sleep may have evolved as a way of relieving vascular stress. The link to depression hints that sleep may a way of dealing with emotionally overwhelming situations.

The research linking a lack of sleep to lower performance across the board is astounding. “Sleep optimizes how your brain works,” Smith says. “There are studies that show sleep helps to regulate emotions and mood. Sleep is also essential for attention, concentration and executive function, which helps us make good judgments, remain flexible and creative.”

On the physical side, sleep is involved in almost every physiological process, says Smith. “Sleep regulates how the body controls pain. Lack of sleep is stressful, causing higher blood pressure and heart rate—and it can also alter metabolism, leading to spikes in blood sugar that can cause insulin resistance. There’s even research to show those who don’t get enough sleep develop the common cold at much higher rates.”

Bottom line? Sleep is one of the most mysterious, overwhelmingly important vital functions living beings have evolved to participate in. You need it. A whole lot. “Even partial sleep deprivation can impair things like cognition, memory, and overall functioning,” Smith says. “You can get used to it, and think you’re doing okay—yet you’re really not performing at your best.”

Everyone could stand to self-evaluate their personal sleep habits, since a whole lot of people are currently suffering from insomnia or what science has termed “Insufficient Sleep Syndrome”—which is exactly what it sounds like.

Here’s how to determine if you’re getting enough sleep, according to Smith:

Smith says this is the best place to start for determining how much sleep you need, because everyone is slightly different. In general, adults need roughly seven to eight hours of sleep per day. Super-far from that? It’s time to make some adjustments. (Check the CDC chart if you have teens, kids or babies in the house.)

Smith says the best way to figure out exactly how much sleep you need per night is to test it out on a week of vacation. “Start by adding an extra half hour to your night, a little more if you feel you need it,” he says. “You should begin to feel the difference in your body, your thinking.” Plus, a good week of sleep can do a lot to help you begin to feel substantially better if you’ve been accumulating a lot of sleep debt.

If you need to caffeinate multiple times throughout the day to stay functional, your focus and attention starts to wane well before working hours end, or you’re excessively sleepy (perhaps actually falling asleep during those afternoon meetings), you’re not getting enough sleep. Every little bit helps. Smith says to try tacking on just 15 or 20 minutes to your nightly sleep—which can add an extra couple hours of rejuvenation to your weekly total.

To sleep in, or not to sleep in? Although advice is mixed, Smith says it’s completely fine to give yourself a couple hours of additional shuteye on the weekends if you’re running on E. “Avoid drastic fluctuations if you have chronic insomnia, though, since routine can help in that instance,” says Smith. Wild swings can toy with your circadian rhythm, and make sleep even harder than it already is, he says.

Keep in mind: it’s also possible to overdo your rest. Get 12 hours of sleep on Saturday or Sunday, for example, and you could be setting yourself up for a restless week. Stick to an extra hour or two max on your weekend days, and you’ll be in better shape for the next five wake-up calls. As with almost everything, maintaining a balance is best.

If you’re meeting these guidelines and still feel tired all the time, be sure to consult your physician. She can help you take a deeper look at your sleep situation and determine the next best steps for you.