Grumpy teenager? Don't blame her – it's all this summer daylight that's causing the problem.
A team of experts says that teenagers find it so difficult to adjust to long summer evenings and lighter mornings that it completely disrupts their sleep patterns, poor things.
Researchers Rensselaer Polytechnic Institute in New York recently studied a group of 16 children between 13 and 14 and found they slept an average of 15 minutes a day longer in winter.
They believe that varying hours of daylight throughout the year affect a pubescent youngster's state of mind. Add this to the cocktail of raging hormones and raucous nights during the summer holidays and it's no surprise that moodiness and fatigue sets in.
By spending more evenings outside, teenagers are exposed to more sunlight, which stops the body producing the chemical nocturnal melatonin, which triggers the body to feel tired. This may put them off going to bed by several hours, because they don't feel ready. And their sleeping patterns are disrupted even further when they are woken by the sun coming up earlier. Roll on winter.
Check out our Surviving Teenagers column.