Hostile mobs spawning in daytime like its night time

After updating Minecraft to 13w33a I am being bombared by hostile mobs in the broad daylight. I have an area cleared for agriculture and at any one time I'm having twenty to thirty mobs constantly spawned and in full hostile mode. There are also bats fluttering everywhere as though I'm in a cave but as I say I am not, I am in the middle of a cleared area (about Y: 64) I hadn't even seen a slime before the update and now five or six large and ten to twelve small slimes are spawning in my immediate area. Its not relevant to the report but I've been killed multiple times, had my crops destroyed and my storage system blown to smithereens scattering my hard found items all over the place.