Funny, but the relative lack of night life (compared to, say, Cancun) is one of the things I like most about Cozumel. Cancun is about the night life; Cozumel is about the day life.
Speak for yourself. I love the night life, I got to boogie! I have been up regularly in Coz till the wee hours.
Seldom do I get to bed until 11 pm.
(And in the winter, it had been dark for a REALLY long time by 11 pm!!, I'm just saying....)