Paul Farquhar
Legend
Except that: “A character can't benefit from more than one long rest in a 24-hour period” so a canny DM can use that RAW to challenge the PCs.
Nor can they benefit from a long rest if they don't get to sleep for 6 hours. So if they are trying to rest in an area that is trafficked by monsters or city watch patrols ("no vagrants, move along") every 5 hours (better make it every 3 hours in case of elves) a long rest is impossible. Note that the encounter doesn't have to be a threat to the party - it just has to wake everyone up.
They also need to have sufficient food rations for every party member to have something to eat.
So it's pretty easy for a DM to restrict rests if they so choose, even without imposing time restrictions (which in my experience tend to be common).
Having said that, I tend not to do that often, and design adventures around a small number of challenging combat encounters, rather than lots of trivial encounters designed to deplete resources. I don't think I have even had as many as 6 combat encounters in one game day.