Rambam says we need 8 hours a sleep; other poskim say about the same, I believe. Getting too little sleep reduces your brainpower, lowers your mood, makes you irritable, suppresses your immune system, and makes you a more dangerous driver.
If you’re getting enough sleep and you still want to stay up later, then taking a shower or getting a little exercise (a few sit ups, push ups, or jumping jacks for example) are two things that help. I would never drink caffeine or exercise at night because I wouldn’t be able to fall asleep even hours later, but everybody’s different.