DAY - lasts from 4:30AM to 7:30PM (in game)
Easier enemies, such as slimes
Birds, bunnies and squirrels appear
Sun will go across the sky
NIGHT - lasts from 7:30PM to 4:30AM (in game)
Tougher enemies, such as zombies and demon eyes
Different music, dark background
Moon will go across the sky
The easiest method to surviving the night is to leave the game running. You can die, but you will just be spawned back at your bed or original spawn point. You can also surround yourself with blocks to protect yourself, or you can build a house and close the doors.