As a matter of fact, you may wish to check out this thread.

It's very likely to be because you are sitting around all day/not working hard etc. I know that if I have a lazy day and sit in watching tv, at night I'm just not tired, But if I go out and mess about doing any active things then I actually want to go to bed and fall asleep almost straight away. But if it comes to it then some brandy/whiskey helps.
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