things that keep me up at night:

[grizzlycrush]

Zombie apocalypse
Robots taking over
Both of those at the same time.
Birds taking over.
Velociraptors.
Zombie velociraptors.
Mummies.
Rabies.
Cyborgs.
Cyborg take over.
Aliens.
Zombie aliens.
Cyborg aliens.
Zombie cyborg aliens.
Vampires!
CYBORG VAMPIRES!
Zombie vampires?
WEREWOLVES.
Zombie werewolves.

EDIT:
ZOMBIE VAMPIRE WEREWOLF VELOCIRAPTORS.
SWAMP THINGS.
ZOMBIE CYBORG VAMPIRE WEREWOLF ALIENS.
CYBORG VELOCIRAPTORS
Phantoms of any kind!

EDIT EDIT:
Dolphins.
Sharks
Radioactive anything.
any combination of all of the above.