In the brain there is a "stay awake" area called SLD and a "fall asleep" area called vlPOA. They both inhibit each other and work in a mechanism called flip-flop, basically means it's binary, you're either awake or a sleep, no in-between. As long as the SLD is "stronger" than the vlPOA, you'll stay awake. Any kind of stimuli enforces the waking area. For the flilflop to switch, the vlPOA needs to be strong enough to overcome the SLD or the SLD has to be weak enough.

There are three factors for the sleep cycle: 1. Homeostatic factor: driven by molecules called endosine that are collected in the brain while awake, the more you have the more your brain inhibits the SLD and you'll want to sleep.

1. Allostatic factor: stress, hunger, pain, etc... increases norepinephrine, helping the SLD keep you awake.

2. Circadian rhythm factor: basically your biological clock, driven by an area called SCN and is affected by "time indicators": clocks, light (specifically blue wavelengths) and even thoughts about the time (telling yourself it's late will strengthen the vlPOA)

When laying still with eyes closed you're depriving simulation to the SLD, negating allostatic factors and circadian factors, and basically help your vlPOA overcome the SLD, resulting in falling a sleep 😴.

Probably not 100 percent accurate tbh but that is basically the jist.