Sleep is a complex process that is influenced by chemical messengers in certain areas of the brain and on an internal body clock that is referred to as circadian rhythm. Sleep is critical to our health and survival and stretches out across 5 distinctive stages. Stages 1 through 4 are called non-rapid eye movement or NREM stages of sleep and the last stage is called rapid eye movement (REM) sleep.

Stage 1: Stage 1 encompasses those first 5 to 10 minutes of sleep, when your eyes close and you begin your descent into slumber. This is the time in which if someone were to wake you, you might ask, "Was I asleep just then?" You might also experience sudden muscle contractions that cause a jerk or a sensation of falling. Catch even a super-quick nap, and you're still likely to improve your chances of remembering something you just learned or memorized.

Stage 2: Stage 2 finds you 20 to 30 minutes or so into sleep, preparing for deeper sleep. It is during stage 2 that your heart rate slows down, body temperature decreases, and your muscles progressively become more relaxed. It's here that you'll hit the pinnacle of the so-called "power nap." A nap of about 30 minutes can offer you the boost of energy and concentration you need to get through the rest of the afternoon.

Stages 3-4: Should you sleep into the 45-minute range, you float into slow-wave sleep. This deep, enveloping sleep state bolsters your declarative memory, the ability to remember facts and explain them. Sleepwalking and bedwetting most commonly occur in the late part of stage 4 sleep.

Stage 5: Once you doze into the 70- to 90-minute range, you're in the realm of dreams and rapid eye movement (REM). Your heart rate and respiration will pick up again, too. Sleep into this stage and you can help to make up for lost sleep and improve your procedural memory, the ability to remember how to perform tasks.

What's interesting is that we don't go through these stages in order all of the time. For example, once you are in stage 4 sleep for the first time, you'll go back to stage 3 then stage 2 before getting to stage 5. And, after stage 5, you will usually move to stage 2 sleep. During a night's sleep, you will cycle through these stages of sleep several times.

How much sleep to we need?

If you don't get enough sleep, you can feel sleepy during the day and your performance (both mental and physical) will suffer. And this can be dangerous if you are driving a car. So how much sleep is enough? Well, that depends on a number of factors including your age, your health, and on how much sleep you've been getting previously. If you don't get enough sleep, you create a "sleep debt" which is sleep that needs to be made up for so the brain can function optimally. Most adults need around 7 to 9 hours of sleep a night.

Written and reviewed by the MediResource Clinical Team