I woke with a jolt at 4.00 am realised there was a light on somewhere, like to sleep in a darkened house, so I must have jumped out of bed quite quickly , I headed for the door and the next thing I was aware of was falling sideways (so effectively at the foot of the bed)I lay on floor for a few minutes until I felt safe to get up. Bloody head hurt for a while. Kept myself awake until I felt OK.
I am presuming it was low blood pressure I am well known for it) and I just moved too fast.
But I am just putting it here, for future reference.