Bedtime rigidity

[adderslj]

I must have been tired last night. Normally I'm pretty mobile in my sleep, occupying the whole of the bed at various times. Last night, I climbed into bed, fell asleep and woke up six hours later in exactly the same position. I climbed out of bed, folded the duvet back, and the bed looked untouched.

Why can't I do that every night?