It could be that I have been in a dimly lit room for a few hours and it is no longer day time, IE the screens auto-brightness is lower then it was earlier in the day and thus more power efficient.
hmm good point! Most likely why then.
How good is the auto brightness? i.e. does it go right to zero when in a pitch black room and ramp up enough to see it easily in the sunshine but not go to full whack.