B777-200 Captain reported a sheet in the lower lobe cabin attendant rest area was nearly ignited where it lay against a light.

Narrative

I was notified by the Chief Purser while in cruise that a bed sheet in the lower crew rest area had been inadvertently dropped against a night/passageway light and forgotten. Subsequently; the heat of the light started to ignite the sheet; and it was discovered due to the odor it made as it started to smolder. When I investigated after landing; the light which had been off for a period of time; was still quite warm to the touch; and there was melted blanket remnant; still adhering to it. I am concerned because there are two lights; one at either end of the crew rest area; that are less than 6 inches off of the floor; and apparently are quite hot while in use. The crew does not appear to have direct control of this light; and subsequently are unable to turn it off if need arises. The sheets being used; are of a synthetic material which; apparently is quite prone to combustion. I feel this is a safety hazard of significance. The sheets should probably be of less combustible material; the lights should have a guard to prevent direct contact with them while in use; and there should be some kind of warning or NOTAM issued to all flight crews; and flight attendants of this potential for fire hazard.