What causes sleep?

[gbfish]

the green crusty gunk that forms on your eyes during the night... what is it and what causes it?

[squiperly]

It's just the natural secretions of the eye which you wouldn't notice through the day, but at night it gathers into the corner of the eye and dries out to form "sleep." The eye needs these secretions to stop it drying out, to wash out foreign bodies such as dust etc and also to fight infection.

[country_boy]

It records your dreams, only we haven't invented the machine to play it back yet!!