Once you start exercising more and get use to it, you'll have to be careful when you do your exercise. I find that anything that blasts my heart rate into high (+140.. guess its all relative though) for long periods of time (more than 30 minutes) will prevent me from sleeping for the next ~2-4 hours. I'll sleep fine afterwards, but during that time, trying to sleep is a usually a lost cause.<p>There are always exceptions though, and it really varies from body to body.<p>The basic advice being 'listen to your body' and 'adjust to how your body reacts' applies to everything to do with exercise.<p>(Also, noticing your note about 20 minutes transit. Strongly consider running/biking to the gym. Dunno what type of weather you have over there, but in anything above -10, you should be fine with a 10 minute run with sweatpants over shorts and a sweater. Just bring one of those satchet bags with shoes, lock, extra jacket or something. You'll save time, and feel badass when you bounce into the gym heart already pounding, and a few randoms looking at you going 'wow, he's ready')