I split my Levemir and adjust the AM dose depending on what I'm doing that day.
There's no real science behind my adjustment really and if I'm perfectly honest, it doesn't always work! It's trial and error for the most part. Reducing my basal for heavy exercise days means that I don't have to drink my weight in carbs to keep my BG's up. I still need to use gels, sports drinks, etc whilst exercising - just not as many.
I haven't noticed a big change in my overnight basal needs, so that's something that is fairly constant.