I have the same problem. Once I write down what I have to do I start thinking about the best way to do it and all the possible scenarios it could lead to.

I've found that having a regular sleep time has helped me the most. I get up between 7am and 9am everyday and go to bed between 11pm and 12am. If I keep to this schedule I find myself naturally getting tired around 11pm everyday. If I don't keep this schedule (which happens often before I rediscover it's benefits) I find I suffer from bad insomnia and often lie awake all night even when I feel extremely tired.