If you've not done much night riding, consider that you may get sleepy and feel colder at some point, traditionally around 4-5am (as grams says, that's usually when the temperature's at its lowest). Also, hanging around waiting for others to catch up or fix mechanicals can make temperature regulation difficult - if you're in a group, it's generally easier to stick to the speed of the slowest rider, rather than playing leapfrog. On the other hand, riding slowly can make you sleepy...