Sleep accounts for one-quarter to one-third of the human lifespan. But what exactly happens when you sleep? 

Before the 1950s, most people believed sleep was a passive activity during which the body and brain were dormant. “But it turns out that sleep is a period during which the brain is engaged in a number of activities necessary to life—which are closely linked to quality of life,” says Johns Hopkins sleep expert and neurologist Mark Wu, M.D., Ph.D. Researchers like Wu are spending many of their waking hours trying to learn more about these processes and how they affect mental and physical health. Here is a glimpse into the powerful (often surprising) findings of sleep researchers—and what they’re still trying to discover about the science of sleep.

All Sleep Is Not the Same

Throughout your time asleep, your brain will cycle repeatedly through two different types of sleep: REM (rapid-eye movement) sleep and non-REM sleep.

The first part of the cycle is non-REM sleep, which is composed of four stages. The first stage comes between being awake and falling asleep. The second is light sleep, when heart rate and breathing regulate and body temperature drops. The third and fourth stages are deep sleep. Though REM sleep was previously believed to be the most important sleep phase for learning and memory, newer data suggests that non-REM sleep is more important for these tasks, as well as being the more restful and restorative phase of sleep.

As you cycle into REM sleep, the eyes move rapidly behind closed lids, and brain waves are similar to those during wakefulness. Breath rate increases and the body becomes temporarily paralyzed as we dream.

The cycle then repeats itself, but with each cycle you spend less time in the deeper stages three and four of sleep and more time in REM sleep. On a typical night, you’ll cycle through four or five times.

Your Body’s Built-In Sleep Controls

According to Wu, there are two main processes that regulate sleep: circadian rhythms and sleep drive.

Circadian rhythms are controlled by a biological clock located in the brain. One key function of this clock is responding to light cues, ramping up production of the hormone melatonin at night, then switching it off when it senses light. People with total blindness often have trouble sleeping because they are unable to detect and respond to these light cues.

Sleep drive also plays a key role: Your body craves sleep, much like it hungers for food. Throughout the day, your desire for sleep builds, and when it reaches a certain point, you need to sleep. A major difference between sleep and hunger: Your body can’t force you to eat when you’re hungry, but when you’re tired, it can put you to sleep, even if you’re in a meeting or behind the wheel of a car. When you’re exhausted, your body is even able to engage in microsleep episodes of one or two seconds while your eyes are open. Napping for more than 30 minutes later in the day can throw off your night’s sleep by decreasing your body’s sleep drive.

Why You Need Sleep

If you have ever felt foggy after a poor night’s sleep, it won’t surprise you that sleep significantly impacts brain function. First, a healthy amount of sleep is vital for “brain plasticity,” or the brain’s ability to adapt to input. If we sleep too little, we become unable to process what we’ve learned during the day and we have more trouble remembering it in the future. Researchers also believe that sleep may promote the removal of waste products from brain cells—something that seems to occur less efficiently when the brain is awake.

Sleep is vital to the rest of the body too. When people don’t get enough sleep, their health risks rise. Symptoms of depression, seizures, high blood pressure and migraines worsen. Immunity is compromised, increasing the likelihood of illness and infection. Sleep also plays a role in metabolism: Even one night of missed sleep can create a prediabetic state in an otherwise healthy person. “There are many important connections between health and sleep,” says Wu.