system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What actually happens when we fall asleep and why can’t we do it on demand?
gpt
The human brain sleeps in cycles, it's called your circadian rhythm. The circadian rhythm controls when you need to sleep and then controls the cycles of your sleep. Some people call it your internal clock. 

The circadian rhythm is influenced by a lot of factors, many related to evolution and the habits of human civilization. For instance, darkness can trigger the need for sleep (because humans have typically used daylight for hunting and working). The reason you can't do it on demand is likely explained by what happens. 

When you sleep, your brain doesn't just shut off. You actually go through two cycles where your brain is still acting, but slowly relaxing. When you hit stage 3 is when you are in "deep sleep". Your brain is using minimal activity. And then, you enter REM sleep which doctors think is like your brain's maintenance time. With all this activity cycles, sleeping and waking on demand would be difficult to maintain. This is why if you wake up during a dream (which usually occur during REM) you tend to feel groggy. 

TL;DR. Your circadian rhythm controls when you sleep. You don't control your circadian rhythm.