Going to bed is one thing but actually going to sleep is another. Did Sheila sleep ? We don't know, nobody does. Sheila may have just lain on top of the bed as her sleeping habits were hit and miss.
Then again, Sheila could have got into bed with her mother because Nevill's slippers were partly under Sheila's bed. He could well have just lain on top rather than wake June who was another light-sleeper.
These are all things we are unlikely to ever know but I suppose it is possible that Sheila lays awake, brooding over the dinner time talk of foster carers and working herself into a state.
Interestingly, and I haven't seen Theroux's film myself, but I heard Barbara Wilson recently gave some support to JB's claim that Nevill and June had been discussing foster care as a possible means of helping Sheila see more of her boys.
I'm not aware of BW ever expressing this before and I'm not sure how reliable her memory can be after all this time but it is interesting and those who have previously dismissed JB's account because Colin would not have allowed it, including myself, should maybe have some pause for thought here and also consider that regardless of Colin's views, June and Nevill may well have stuck their noses in out of genuine concern and rather than see that this was well intentioned, Sheila may have heard the word "fostering" and gone down a dark path.