Putting a laptop to sleep is not damaging to it but if you don't plant to use the laptop for more than 3 hours you should shut it down to prevent it overheating.
Laptops and other computers are designed to be put in sleep mode to conserve energy and make them easy to start back up.
But to save the laptops lifespan it's best to shut the laptop computer down completely if you're gonna leave it alone for 3 hours or longer.
Such as overnight it's best to shut the laptop completely off and then turn it back on in the morning.
Laptops are crammed tight with the heat sink, CPU etc and can overheat when left on overnight and although they are usually okay left on overnight in sleep mode it's best to shut the laptop off.