Is it better to exercise in the morning or at night?

Walking during the day means that your body must use more energy to keep you cool, since your body won't need to cool you as much during the night you'll burn slightly more energy during the day. The same goes for cold, the colder the more energy goes to heating you up. Also, walking during the day is better for your eyes, since when walking at night you have to deal with the sharp contrast between the darkness of night and the brightness of city lights. If you're walking to build muscle you probably won't notice any difference, if you're walking for cardio reasons go during the day, the heat will cause you to use more energy and will (psychologically) give you the will to even be out there. Honestly, there's not really any differences other then really minute ones. Walk when it's convenient, but do it regularly. Try to walk at the same times every day/night.