If you're very busy and stressed, it's common not to have the energy/enthusiasm to get any exercise in. But it's common for sleep problems to be caused by lack of exercise. The problem then is that although you're mentally tired, your body hasn't had any work to do so you're physically craving something to work your muscles.

Don't do strength-building exercise if you're having trouble sleeping, because it kicks in all the adrenaline and you'll never sleep. But lower-rate cardio work - gentle jogging, brisk walk, etc - is good. Just some push-ups followed by a bit of stretching helps me.

Graham.