Ever since I've known W she has been a big sleeper, I think it has something to do with her parents allowing her to sleep in as long as she wanted at the weekends. I remember that MIL was not allowed to use the vacuum cleaner at weekends until W had woken up (sometimes at 2pm in the afternoon) or there would be hell to pay.
Anyway about 3 years ago W was going through a period of poor health. Her breathing was affected and it got so bad that just walking up the stairs would leave her out of breath and needing to rest. Anyway after extensive test (and I mean extensive) W was diagnosed with COP (known as BOOP in the USA), this is a degerative lung condition where the lungs deteriorate to the point where they can no longer pass oxygen into the blood, if not caught in time the end result is death unless a lung transplant is made. In W case, the damage to her lungs left her with 40% capacity which she has managed to expand to 60% with reasonable exercise. The spread of the disease was controlled with steroid drug treatment, which W is now tapering off, but she still does need lots of rest after any exertions.
So where I have mentioned in the past that W always falls asleep, part of it is due to her condition, part of it is due to the way she was allowed to grow up, the two are not connected. Oh and yes I have mentioned the use of oxygen to W, in fact when her condition was at it's worst I bought canisters of oxygen, the type that sportsmen use, the idea was when she was out walking and got out of breath she could take a quick zap to boost her, but in her normal drama queen way she discarded them after a couple of goes.
At the moment W is fine but she accasionally laments her loss lung capacity which does restrict the amount of exercise she can do, but as I sometimes tell her she is more able that some people with full lung capacity.