This is all excellent advice. Many thanks, everybody. Going to look into combining the alternate rest rules with creating much more of a time crunch to see how that changes the nature of their combat encounters overall.
5e is a resource management game (fundamentally). Smites, spell slots, hit points, HD, divine channel, Lay on Hands, Sorcery points, Ki points, action surge, superiority dice etc etc.
A 9th level Paladin has 4/3/2 slots for smites and spells, 60-70 odd HP, 45 HP worth of Lay on Hands, and 1 channel divinity per short rest plus 9 HD for short rest healing.
Making your players deal with multiple encounters between long rests (whenever those long rests happen) is a major part of 5E's mechanical balance.
The math seems to largely even out at the 6 (or so) Hard-medium encounters per Long rest, with a Short rest every 2 encounters or so.
Paladins, Barbarians, and all Full Casters are 'Long rest' dependent (all their resources recharge on a Long rest). If you dont police the Adventuring day, they'll naturally try and Nova those Long rest resources (dumping high level spell slots, raging at will, smiting on every hit) without much thought, and will then fall back to rest overnight.
Dont let them.
Warlocks, Fighters and Monks are Short rest reliant - their spell slots, actions surge, Ki points, superiority dice, second wind etc all come back on a Short rest.
If you compare Paladins with Fighters of the same level, you'll find they balance out roughly at that 2 short rest per long rest ratio.
More encounters between Long rests mean your Players will have to ration those smites and rages and high level flashy spells for when they're really needed rather than just 'mashing buttons'. It adds to the game, and also makes those flashy high level spells, and 'long rest' powers really shine when used.
Importantly it also maintains balance. The worst thing you can do is do nothing re resting, and dial up encounter difficulty as all this does is force the players to Nova (and then fall back and rest) in order to survive the encounters. Class balance goes totally out of whack, and the game becomes glorified rocket tag.
