It’s right around the preteen years that bedtime starts to become a battleground. By the time kids are teenagers, it’s often flat-out war, with kids wanting to stay up late, and parents begging them to go to bed earlier. A late-sleeping teen is often chalked up to adolescent contrariness and desire to flout rules, but there’s actually a good reason she wants to stay up late: Her brain is telling her she’s not tired.

The brain of a sleeping teen

The brain has two systems that control what time we go to sleep and wake: The internal biological clock, which responds to light in our environment and signals sleepiness at nighttime and alertness during daytime, and the sleep-wake cycle, which controls our active hours, switching us on in the morning and increasing our fatigue as the day progresses.  In adulthood and childhood, these two systems work in tandem so at night, when it’s dark out and we’ve been awake for some 10 to 12 hours, our brains signal our bodies to prepare for sleep.

In adolescence, however, these two systems work against each other. Called a phase shift delay, this natural stage of growth and development is why preteens and teens are known to be night owls.

At around the age of 10, as a child’s body prepares for adolescence, her biological clock changes. Instead of signaling her body that it’s time for bed at around 9pm, as it used to, her brain now sends that signal three to four hours later. Moreover, at 9 to 10pm her brain is actually making her body feel more alert, stimulated, and excited. Rather than starting to tire, the typical preteen or teenager actually feels great – wide awake and at a high. It’s not until around midnight that the biological clock will kick in and the preteen or teenager will begin to tire.

In a perfect world, this wouldn’t be an issue – we would all sleep when we are tired and wake naturally. But most preteens and teens face early school start times, increasing loads of schoolwork, and technological stimulation that can inhibit sleep hormones; combined with the physiological changes of puberty, most kids rack up huge sleep deficits.

Playing catch-up

According to several studies, the average teenager should be getting about 9.25 hours of sleep each night – but instead he is getting roughly seven. Each night, as the teen gets fewer than the optimal amount of sleep, he builds up something called a sleep debt of roughly 10 hours each school week.

Which leads to the other side of teen night-owl behaviour – what parents often perceive as laziness when kids attempt to make up their sleep debt during the weekends by sleeping nine, 10, 12 hours every night, often from late at night until late morning or even early afternoon.

In theory, this catch-up seems like a good idea. But there are two problems with sleep binging: First, sleeping those extra hours over the weekend still isn’t enough to work off such a large sleep debt, and second, going to sleep later over the weekends, say around 12 to 2am, reinforces the brain late-sleeping schedule and makes it even more difficult to go to sleep earlier during the week. In other words, come Monday, the brain that has become used to sleeping at 2am suddenly has to be awake at 7am for school.

But the solution isn’t to live in a perpetual state of sleep debt until the end of adolescence. Sleep deficits can have significant consequences for preteens and teens, affecting their mood, performance, reactivity, and most importantly, their learning.

Sleep and learning

The teenage brain is primed with an incredible ability to acquire and learn new material quickly, and quality sleep is a key part of that process, predicting whether a teen improves or regresses in new skills and abilities.

While researchers are still studying the exact link between sleep and learning, this is what they think is happening in the brain of a sleeping teen: New knowledge during the day is stored in a part of the brain called the hippocampus, a short-term memory hub. Once asleep, all of that new information leaves the hippocampus and gets transferred to the parts of the brain responsible for higher order thinking, where the new knowledge gets categorized and networked to other, previous knowledge.

So, imagine an awake teen learns the Pythagorean theorem – a² + b² = c² — to find the length of the longest side of a triangle. In a sleeping teen, that new information will leave short-term memory storage and integrate with existing geometry and algebra knowledge networks, rooting it in the understanding that all triangles have three sides, that a, b, and c represent numbers, and squared means a number multiplied by itself. This is how the teenager builds upon previous knowledge – by growing existing networks of information during sleep.

Then, as the sleeping teen slips deeper into sleep, a replay phenomenon takes place. The same parts of the brain that were activated when the new information was first imbibed activate again, as if the teen is relearning the information. This stage of sleep is particularly important for more complex cognitive learning, like those maths problems. Short-changing it can cause a nearly 50% gap in performance, according to a study by Carlyle Smith of Trent University, who examined the difference in academic performance between students who got enough and good quality sleep and students who had sleep deficits.

The biological change to adolescent sleep patterns rebalances in the mid-20s – but that’s a long time to wait and a lot of learning a teen could be missing out on. In the meantime, it might help to get your teen on board by explaining to them the role sleep plays in learning and performance.

And while you may not be able to change school start times, you can help your teen to set up a routine of relaxation and sleep (pro tip: avoid devices like laptops and phones right before bed and consider afterschool naps) that they feel comfortable sticking to even on the weekends.