Just to add my greetings Heather Robinson, and yes you will be able to check in any time from 05:00 hrs in the morning from T3, T5 or indeed LGW ( a bit earlier there on some days). No-one will blink at that either, it's not a big ask or anything like that. But welcome to the forum and welcome to Flyertalk as well.

The only thing I would caution, beyond the obvious risk of boredom for such a long stay, is that by the time you get to Atlanta you may be past the point of exhaustion, it's the sort of thing that can hit you for several days running Just something to bear in mind.