system
Given the following post, respond as if you're commenting on Reddit:
human
eli5 Why is it so easy to fall asleep when your trying to stay awake when your so tired and watching a movie but so hard when you actually want to go to bed ?
gpt
All the stimulus created while you're awake makes your brain want to shut down and sleep.  When you're lying in bed trying to sleep all the stimulus is gone and your brain will be processing the day and keep you awake.

Not the scientific explanation,  but close enough.