At many hostels, every time you leave the dormitory, the door will lock behind you. This means that when you wake up for the 3am wee, you need to take the key with you, otherwise you will find that the only way back into your room is by knocking loudly and hoping that your room mates not only hear you, but will actually come and open the door for you, which they might not if you’ve already annoyed them by waking them up as you tried to climb quietly down the impossible bunk bed ladder and make your way out of the room. I always sleep with a variety of useful things under my pillow, including my key, which isn’t always comfortable but at least I know where it is when I need to find it in the dark.