Might be something to do with what you eat. Do you drink a lot of coffee? Try drinking tea. Or do you keep on doing something beyond the time your brain shuts down because it's had enough?
Do you get out? - I don't mean to the supermarket or the cybercafe, I mean out to the park or somewhere less 'organised', such as in the woods. If you live in a big city it's going to be hard to get out to woods and heaths etc., that's why you've got parks.
To the north west of London is Hampstead Heath, a great area to look down on 'the Smoke' (the name non-Londoners - such as me - give the capital). Or a bit further out is Richmond's Deer Park (near where Mick Jagger lives), and another way out, past Wandsworth is Wandsworth Common.
Although I live in a numbered district of London, within half an hour on the Underground (Subway to you) I can be out in Essex, on my way uphill into Epping Forest and on to one of the tea huts at High Beech. Or, half an hour's walk away is Wanstead Park, technically part of Epping Forest. You'd be amazed at how even that half hour's walk can de-stress the brain.