To me, there’s nothing better than waking up after a full eight hours of sleep. Coffee could never replace the feeling of being naturally refreshed, alert, and rested. Some people claim they function on only a few hours, while others must get their complete night’s rest. No matter if you’re a night owl, early bird, or sleeping beauty, sleep is an essential part of human survival; if we don’t sleep, we die.

While it may seem simple, sleeping is actually incredibly complicated. There are aspects of the sleep and dream cycle that scientists and specialists still struggle to grasp. Here are the explanations to some of the biggest questions about sleep and dreams:

Why do we sleep?

To be honest, no one is entirely sure why we sleep. Some believe that sleeping is a way to recover from the events of the day. However, sleeping doesn’t actually save much energy. The most widely accepted reason is that sleep keeps the brain in functioning order. If you’ve ever pulled an all-nighter, you know that the next day you may feel extremely groggy, irritable, and unalert. Concentration and attention span are affected even after one night without sleep. Having a sharp and alert brain was once essential for early human beings, whose daily lives would include trials of survival that modern beings don’t experience. It can be incredibly dangerous to not sleep for many hours. As explained in a BBC article, “17 hours of sustained wakefulness leads to a decrease in performance equivalent to a blood alcohol level of 0.05% (two glasses of wine). This is the legal drink driving limit in the UK.” Sleep deprivation can also lead to a lack of rational decision making and even hallucinations.

Why do we sleep at night?

Back in the days of cavepeople, it was far more dangerous to be out in the dark than it was to be outside during the day. It’s much easier for humans to see when it’s light out, and being out at night increases the risk of harm (falling, tripping, being attacked, etc.).  Similarly, animals who are nocturnal (sleep during the day and wake at night) are more likely to be attacked or preyed upon during the day. If early human beings had better night vision, we might sleep during the day instead!

What are the stages of sleep?

There are four stages to the sleep cycle. Once stage four is reached, the cycle begins again.

1. Light sleep – during this stage, the person is half awake and half asleep. The person can be awakened easily in this stage. Their muscles begin to relax and might twitch slightly.
2. True sleep – this stage typically occurs 10 minutes into the sleep cycle, and lasts for around 20 minutes. The heart and breathing rate begins to slow down.
3. Deep sleep- while similar to REM sleep, this stage is when the brain begins to develop delta waves. Heart rate and breathing is at its lowest.
4. REM sleep – REM stands for Rapid Eye Movement, named because our eyes dart back and forth under our eyelids during this period. This typically occurs 70-90 minutes after falling asleep. One might experience three to five REM episodes every night. The brain is still very active during this stage, and it is where dreams occur. This is also the stage in which people are most likely to experience bed-wetting, night terrors, or sleepwalking. The first episode typically lasts 10 minutes.

Most people sleep between five and a half and 11 hours a night. Whether you’re more like a python (they sleep up to 18 hours) or a giraffe (only one and a half hours), you still should try and get as much sleep as you can. The world record for the most amount of time without sleep was 11 days, set in 1965. There’s a reason that Randy Gardner still holds this record today.