Prof Derk-Jan Dijk, from the Surrey Sleep Centre, said there would be no harm in trying to make bedrooms darker.

Every year the entire centre of Phnom Penh is snarled with hundreds of thousands of rural visitors, many of whom sleep in temples, schools and sidewalks during the period of the festival.

Your brain is most alert one to two hours after waking, according to the Sleep-Wake Disorders Centre in New York.