Don't get me wrong, I do believe that masks on board - generally - are helpful. But in some situations, like when you are sleeping having a pod around you, I don't think a simple surgical mask adds much. I've not flown much long-haul the last two years (Only 3 trips; One of each KLM, Air Europa and United) so my experience may be a little less extensive compared to some others. However on every single of these flights I've noticed some people putting their masks back on when the cabin lights are turned on. Personally I switch to a mask with exhaust valve for easy breathing while sleeping. Not according to the rules, but like I said...when you are lie-flat with a pod around you... there would be no difference anyway (Compared to a simple surgical mask).

When flying Economy (which so far I've been able to postpone thankfully) I'd probably not done the same, and make sure to wear a proper FFP mask throughout the flight.