It was one of the worst pieces of advice I was given in the early years of being ill - to maintain "good sleep hygiene", ie not sleep in the day. Obviously there's basics about looking after the body clock and it's not good to stay awake all night on caffeine and computer games, but daytime sleeps can be restorative when you are ill and we need as much decent rest as we can get.
I suspect that the whole sleep hygiene story is another one of those medical rules of thumb that applies to a subset of healthy people, but gets applied to everyone -- far beyond the existing supporting data. It's no mystery that guidance that applies to healthy people can be completely wrong for people who are ill. It's not even difficult to comprehend, but too many medical people don't even try to think. They just apply their little nuggets of wisdom without understanding where they come from or what the limitations are.
I full well understand that people who get into bad habits, or perhaps who sleep during the day due to apathy or depression, might not
need the sleep and therefore could be disrupting a healthy sleep pattern by napping during the day. I don't see why it should be assumed that everyone who naps during the day falls into that group.
OTOH, I wonder if not sleeping/resting during the day is, in fact, the 'natural' human pattern. Sure, daytime napping doesn't fit the 21st century office-worker lifestyle, but is it really true that primitive people in hot climates didn't sleep during the hottest part of the day? Isn't there a lunch and nap routine in some farming cultures? Yes, it appears natural that the longest part of our sleep is during the night, but why did we decide that means no sleeping at all during the day, especially for people who consume a lot of energy during that time?