I think the early morning arrivals are for businessman - so they can hop straing into meetings. I also would imagine slots have something to do with it.
I have landed at LHR at all hours of the day and find the lunchtime/early afternoon flights are the best. You arrive, go home, have a shower, sit down with your mates, have a meal, then to the pub around 10:30 you are so tired you go home, and sleep the whole night. No jet lag - that how real players do it.