It's your brains way of saying, "I don't give a shit about what you have to do, or what appointments you have... I think you need more sleep, because I know what's good for you." The body is a machine, without downtime, it starts to break down. The brains basic programmed functions (I'll call it the BIOS, to make neonerd happy) includes the demand that the body gets rest, when it thinks that it needs it, it makes you feel tired. It makes the thought of a nice warm bed and fluffy pillow sound VERY GOOD. Also, the brain does need a certain amount of time to regain its motor and cognitive functions. If you're woken in the middle of a deep sleep cycle, you'll probably be pretty flustered and confused, the brain doesn't function quite right. I'm assuming this is due to the brain automatically shutting off a lot of neurological functions when you're in that deep sleep cycle. There have been times that my alarm has woken me up way too early, and in my confusion I mistook my alarm for a computer, and got extremely irritated when I couldn't find the mouse to shut it off. At the time I had one of those alarms with the big blue backlit LCD
On a related note, for the last 6 days I've had very long and rememberable (but senseless) dreams. I'm wondering how long I can keep it up.