What's up with Minecraft mob spawning?

So I'm playing MC and about readying to digging for gold, iron, and precious diamond. I get all my tools ready and get a stack wood, bread, and torches along with a clock and a stack of wool so I can make beds and sleep through the night so all the creepys don't kill while I mine. Well I go down a near-by caves and am about half way down when I'm surrounded by zombies skeletons and freaking creepers. I was confused because it was still day-time, but assumed they spawn some other night when I didn't get to bed fast enough, but over-whelmed I set the difficulty to peaceful, watched them all despawn and set it back to easy. Well I go further and I met a zombie while it is still day time, but then the one zombie becomes five zombie, two skeletons, and oh so hugable creeper. Pause, peaceful, despawn, easy, zombie. I keep getting overwhelmed by mobs, setting it peaceful, and then turning it back only to get overwhelmed five minutes later. Now usually I have no problem dying like the adventurer I'm but all of this happen during the day, seriously my clock would be on high noon and skeletons snipe me from behind. I thought monster only spawn at night, but they are clearly spawning during daytime. So what's going on?

If its dark in the cave that's why

When your in a cave mobs spawn because its dark. Put torches down everywhere and the chance of them spawning is not as much.

Monsters spawn in dark places, they spawn on the surface at night because it gets dark, so long as you light up the place your at monsters won't spawn there.