John Watson sleeps with his shirt off. He didn't used to, but Sherlock worked out that if he adjusted the radiator thermostat in John's room by four degrees, John would remove his t-shirt in his sleep rather than kicking the covers off. Sherlock finds John more aesthetically pleasing like that, possibly as a counter measure to the bulky jumpers he wears during his waking hours. John Watson also sleeps the sleep of one used to sleeping in a Warzone. He never wakes up when Sherlock watches him.
Sherlock doesn't sleep. Well, he does sleep, just not like normal people. When he does finally decide it's time to close his eyes and visit the Land of Nod, he passes out, sometimes for days. So whilst John is sleeping and thinks Sherlock is sleeping, he is in fact watching. Watching the rise and fall of John's chest, observing the minutiae of John's body as it rests. Sherlock likes to watch John, but he doesn't really understand why. That tiny detail of what it is that keeps him hiding in the shadows, still and hardly breathing, has escaped the processes of his enormous brain.
Mycroft does sleep. Precisely three hours a night from 4am until 7am, national emergencies notwithstanding. And he wears dark blue silk pyjamas. Not that anyone but his Tailor and his Housekeeper would know that. And Mycroft likes to watch as well. He likes to watch Sherlock watching John, wondering if one day his brother will ever understand what power he has allowed John Watson to wield over him. Mycroft had a discreet camera installed in the smoke alarm above John's bed. It amuses him greatly. So does the good Doctor's unfortunate choice in brightly coloured underwear.
John has no idea of the audience he has whilst he sleeps. And what he doesn't know won't hurt him. Sherlock doesn't want to hurt John.
Sherlock has no idea of the audience he has whilst he watches. If he did, he would probably want to hurt Mycroft very badly.
Mycroft doesn't care whether John or Sherlock are aware or not. He's grateful for the small mercy of knowing they are both safe for at least a small part of the night. It helps him sleep.