There is no way to manually activate it. You will either have to change the bed time to 8 pm or afterwards manually change the time that Garmin think that you fell asleep if it doesn't find the correct time.
You can change the sleep hours anytime, or set them sufficiently large (even round the clock, for example 00:01 to 23:59) so that the sleep is detected automatically whenever you go to sleep. Do not forget to sync the watch, after the change of sleep hours.
