system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't we just fall asleep whenever we want?

I want to sleep, I tell my brain I want to sleep, my body needs sleep to function properly, and yet I'm lying awake at night frustrated that I can't fall asleep when I need to get up for work in a few hours. Why don't we just flip a switch in our heads and fall asleep?
gpt
Our sleepyness depends on several factors:

- Sleep homeostasis: In layman terms, it means internal 'sleep pressure' - depends on how much sleep you have had recently, the quality of sleep etc. e.g. you're likely to feel more sleepy when you're sleep deprived.

- Circadian rhythm: The internal rhythm of the body, also known as the 'Biological clock'. Depends on things such as what your body is used to, levels of Melatonin (a hormone in our brain that regulates sleep), Core body temperature (lower help sleepiness) etc.

- External stimulants: Most common reason for not being able to fall asleep in our times. Things such as Caffeine, other stimulant drugs, too much excitement about something, worries etc prevent the sleep from setting in.

- External depressants: Such as sleeping pills, alcohol etc which act to stop/slowdown certain brain functions.

The combination of all these factors decide how easily you can sleep. So, it varies for different people at different times.

Also important to know that sleep is still not fully understood, so we can look forward to more details as scientists' understanding of sleep improves. 

EDIT: Added a bit more details.