Also, the control group is not someone reading a book with a lamp, but someone reading a book in a dark room without a lamp. So really it boils down to 'light' or 'no light'.
So sleeping? Not sure how you read in complete dark…
"very dim room"...
The control group was allowed to use reading lamps and adjust lights and seating position to their liking.
Can you point me where that is described? All I find is "(ii) reading a printed book in the same very dim room light for ∼4 h before bedtime for five consecutive evenings", highlight mine.