Which is correct fall asleep or fell asleep?

To fall asleep is an expression, a figurative way of saying to go to sleep. However, we don’t say to fall to sleep in English, instead, you can say to go to sleep. When can we use fell asleep? If you want to talk about a past event you can say; I fell asleep last night/yesterday/last month as fell is the Past Simple of the verb to fall. When can we use fall asleep? If you want to refer to right now, your sleep routine or something that you do every night for example, you should use fall asleep as fall in the Present Simple of the verb to fall. For example, I always fall asleep after reading a chapter of my book. (this is something that happens every night and is part of your daily routine.)