Picture from eHow: How to Wake up Early to Bike to Work (http://www.ehow.com/how_2192302_wake-up-early-bike-work.html)
Sometimes, we can’t resist from sleeping while at other times, we just can’t seem to sleep. What actually wakes us up is also difficult for the scientists to explain.
When we do different types of activities, we use up a lot of energy. So, the brain and the body need rest, which is provided by sleep.
The most probable reason is that while we are asleep, our body feels hungry, cold, damp, experience fear, etc. These emotions stimulate our brain and we wake up.
Below Sleep info from Wikipedia (Read More):-
Sleep is a naturally recurring state of relatively suspended sensory and motor activity in animals, characterized by total or partial unconsciousness and the nearly complete inactivity of voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and it is more easily reversible than hibernation or coma. It is observed in all mammals, including humans, all birds, and many reptiles, amphibians, and fish. In humans, other mammals, and a substantial majority of other animals that have been studied (such as some species of fish, birds, ants, and fruit flies), regular sleep is essential for survival.
http://en.wikipedia.org/wiki/Waking_up
