As others have said, one missing feature in the (otherwise fantastic) T5 lounges are dedicated snooze zones.

Galleries 1st is rather bright and lacks anywhere good to sleep IMO.

Best option I'd say would be to eat (if need be) at the Galleries 1st then head upstairs to the Galleries Club where the cinema room is nice and dark, with comfortable long sofas you can lie flat on, plus it never seems very busy. On the few occasions where I've needed to sleep in T5 it's done the trick.

Or head to the Sofitel, although it'll be rather pricy for not a lot of snooze time one you factor in getting out of T5, checking in, etc.