Why do I shake before I fall asleep?

Okay, first please note that this does not happen at night time, but in the aftertoon around noonish when I go back to sleep because I get up very early for work. When I lay down and start to feel me getting tired and comfortable I start shaking, as like an earthquake. It feels like my bed rumbles and my body at the same time but I don't know what it is. It never happens at night and I can't see why over 50% of the time it happens this is occuring. I don't think it would be a constant timed mini quake since they are rare anyways where i live. I just dont know if its my body or something else causing it. I run a fan next to me, but i always have that on night or day. Anybody have this happen to them, its hard to explain?