About 64 per cent of high school students studied complained of excessive daytime sleepiness. (iStock)

Teen drivers were twice as likely to have a crash if they were sleepy behind the wheel, a finding that shows adolescents need to learn the dangers of sleep deprivation, Italian doctors say.

The study in Monday's issue of the Journal of Clinical Sleep Medicine looked at 339 licensed high school students with an average age of 18.4 years in Bologna, Italy. Eighty of the students had crashed at least once, and of these, 15 per cent considered sleepiness the main cause of the crash.

The results showed adolescent drivers were 2.1 times more likely to have had a crash if they experienced sleepiness while driving and were 1.9 times more likely to crash if they reported having slept poorly before getting behind the wheel.

The only effective countermeasure to drowsiness is to stop driving immediately, pull over to a safe place and nap for 10 to 15 minutes, said the study's lead author, Dr. Fabio Cirignotta, a professor of neurology at the University of Bologna.

"Commonly used countermeasures to fatigue, such as opening the window, listening to the radio or drinking a coffee, are known to be short-lasting and, essentially, useless," Cirignotta said in a release.

"If a subject perceives sleepiness, he or she would probably already have a reduced performance at the wheel, and nobody can safely detect the real instant when sleep is starting in order to stop driving at that time."

In the study, students filled out questionnaires anonymously about their lifestyle habits, such as drinking caffeinated beverages or alcohol, use of tobacco and drugs, nighttime sleep habits, symptoms of sleep disorders and daytime sleepiness.

Another set of questions assessed driving habits such as speeding, and sleepiness at the wheel based on how often and when participants drove, particularly at night, and the students' coping methods for dealing with sleepiness while driving.

Compared with sleep research showing adolescents need nine to 10 hours of sleep per night, the results showed students suffered from chronic sleep deprivation — something to which adolescents are physiologically predisposed, the researchers said, because of maturation changes in their sleep-wake cycle.

The circadian timing system or daily clock gets reorganized to push sleep to occur later in adolescents, melatonin, a hormone produced by the body to induce drowsiness gets secreted later in the evening during the teen years, and social and academic demands encourage adolescents to stay up late.

Participants said they needed an average of 9.2 hours per night, but the students said they slept only 7.3 hours on weeknights.

About 58 per cent of the students said they tried to catch up by sleeping nine hours or more on weekends, which previous research suggests won't work to compensate for chronic sleep deprivation.

The students also commonly reported sleep problems:

• 64 per cent complained of excessive daytime sleepiness, such as falling asleep while watching TV, listening to music (49 per cent), reading or studying (32 per cent), and travelling (46 per cent.)
• 40 per cent said they had trouble waking up in the morning. Most subjects, 59 per cent, said they would prefer to wake up after 9:45 a.m.
• 19 per cent reported bad sleep.

Of those studied, 58 per cent were men. The researchers found a 3.3 times higher risk of car accidents in men.

Smokers were a 3.2 times higher risk of car accidents. The team suggested tobacco might be used as a way to fight off sleepiness and its use might provide an indication of how unhealthy a person's lifestyle is.

"Poor sleep quality, together with sleepiness at the wheel, significantly increase the risk of car crashes in our population," the study's authors concluded.

"Inadequate sleep hygiene could have negative effects on daytime functioning in adolescents, therefore, education programs on sleep issues and the dangers of sleep deprivation targeted toward young people are warranted."

The researchers gave the example of a two-hour interactive course that was given to another group of 540 Italian high school students. Three months after the course, students showed they retained the information on the need for sleep and the consequences of lacking sleep over time.

The study was funded by the Italian Ministry of Health and Italian Ministry of Education.