Sleep deprivation and adolescence often go hand in hand and it’s no wonder why: Many teens hit the sack after midnight and are sitting in homeroom before sunrise. Experts have long warned that sleepless nights predispose teens to learning disabilities, emotional problems and even car accidents. Now, a new study finds teens aren’t necessarily to blame: There’s a real biological reason for their struggles with sleep.

Our sleep and wake cycles depend partly on the release of a hormone called melatonin that’s produced in response to sunlight. Scientists noticed that today’s teens are indoors most of the day and wondered if the lack of sunlight disrupts their sleep.

To find out, they asked a group of eighth-graders to wear large orange sunglasses that blocked out the blue, short-wave light of dawn. Eleven students agreed to wear the glasses from the time they woke up until the moment they returned home from school in the afternoon.

The students came back to the school every night for a week so scientists could measure the amount of melatonin in their saliva at different points in the evening. The study found that students who wore the glasses experienced a much later surge in the sleep hormone, melatonin, and were more likely to toss and turn for half an hour longer than their friends before finally falling asleep.

The researchers hope their results will serve as a wake-up call to schools, encouraging them to design buildings with more skylights and windows. After all, a good night’s rest begins with the first light of day.