Actually not entirely bizarre.
In an enclosed environment with little ventilation the levels of carbon dioxide exhaled would quickly (relatively) build up.
And estimates of available oxygen in an average sized (UK) bedroom, and average respiration rates, indicate that an average person would start struggling to breathe properly at any point after about 3.9 days, and by about day 12 there would likely not be enough oxygen to survive any longer.
Now that is probably a bit longer than people would expect, but that is based on averages, so you could easily imagine the extreme ends of the data being shortened somewhat if you account for, maybe, smaller rooms, more than 1 person in the room, 'heavy' breathing, etc.