Natural ways to deprive the party of at least long rests?

I’m starting to get sick of the ambush while they sleep trope.

Are there any better ways to get to “you can’t long rest here” for some reason?

My rest economy is annoying because I can’t string together enough combats to make fighting feel dangerous without them having the chance to get everything back.

EDIT: Thank you for all the timer recommendations. Now I need to figure out how to give them a time constraint while not railroading them down paths.