How do humans go to asleep?

I believe (though I have no idea if it is proven or not) you fall asleep when you stop thinking. That's why I think it is hard to fall asleep when you have a lot on your mind (ex. day before or after any holiday) Or it could be difficult when you are sleeping somewhere that is not comfortable you are thinking about that.

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answer 2 When you go to sleep, most of your external senses are turned off - or at least down - sound is a notable exception; and your brain if free to process the bits of today's information that didn't make sense. Initially you go into a shallow sleep; then a deep phase, in which muscles are very relaxed; then you move into a shallow phase of sleep called REM sleep. You will usually repeat this cycle several times each night.

REM [Rapid Eye Movement] sleep is marked by - you guessed it - and in this phase of sleep your brain is 'watching' lots of different activity.

The eyes move rapidly though the lids are closed, and the electrical activity of the eye muscles may be easily recorded. It is in this phase that dreaming takes place, and if you wake someone immediately after REM sleep, they will be able to record their dreams. All people go through REM sleep each night - often several phases of it - but unless you wake soon after REM phase, you will probably not remember it.