I said after 4 hours, but I can actually still function after only 2 hours of sleep. Function being a relative word. I can make lunch, but realise I left the margarine in the grocery cupboard, right next to the milk, and put the Coco Pops in the fridge with the peanut butter, and sugar. I feel dangerous whilst driving because I feel like a zombie. At work, I forget all my passwords, and process journal entries the wrong way round. I send "joke" emails to the wrong people because their name starts with the same letter as my friend. I've even forgotten my name on more than one sleep deprived occasion.
So, I CAN function on only 2 hours sleep, but 4 is better.