They CAN’T go to bed earlier: how is teen sleep different from adult sleep?

When we talk about sleep deprivation in teens, all too often we hear: “they should just go to bed earlier”. Given a 7:35am start time for the Masconomet Regional School, and buses picking kids up between 6:30 and 7:00am, most of our teens needs to wake up around 6am. To get the 9 hours of sleep that they need to be healthy (yes, teens need more sleep than adults), they would need to be asleep in bed by 9:00pm. Many teens can probably push their bedtimes back 20 or 30 minutes, but to 9:00pm? That’s unlikely.

It’s difficult to understand just how impossible it would be for teens to fall asleep at this time without knowing more about sleep in general: what makes us feel alert, why do we get sleepy, when is our natural bedtime, and how are teens different? This article will help you understand the forces that affect sleep and how those forces are different for teens.

What is the sleep-wake cycle?

To really understand how teen sleep is different from adult sleep, you must first understand what drives the adult sleep-wake cycle. All humans, once we wake, begin to accumulate Homeostatic Sleep Drive, for each hour of the day that they are awake. This is thought to be caused by an accumulation of chemicals in the brain (adenosine and erer somnogens). As these chemicals build up, over each hour of the day, we get more and more sleepy.

The image below shows the Sleep Drive (in blue) building up throughout the day, starting at about 6am, when the average adult wakes. At 10pm, when the average adult falls asleep, the Sleep Drive begins to fall, as chemical toxins are cleared from the brain. Most adults need 8 hours of sleep to be healthy – some are owls, who fall asleep later and wake later and some are larks, who fall asleep earlier and wake earlier. This chart shows the average adult.

The Sleep-Wake Cycle of Adults (Alerting Signal, Sleep Drive, Circadian Dip, Second Wind, Circadian Nadir), Start School Later Masconomet
The Sleep-Wake Cycle of Adults, Sleep Drive (click to enlarge)

If the Sleep Drive was the only thing governing the sleep-wake cycle, we would spend every waking hour getting more and more sleepy, until we finally succumbed to sleep. However, there is more at work than just the Sleep Drive. All humans (in fact, all mammals) also have an Alerting Signal (sometimes also called the Circadian Wake Drive), which is their own natural biological alertness cycle. The Alerting Signal works to counter-act the Sleep Drive during the day, keeping us alert and awake during daylight hours. The chart below shows the Alerting Signal overlaid on top of the Sleep Drive. As long as the Alerting Signal is greater than the Sleep Drive, we find it easy to stay awake.

The Sleep-Wake Cycle of Adults (Alerting Signal, Sleep Drive, Circadian Dip, Second Wind, Circadian Nadir), Start School Later Masconomet
The Sleep-Wake Cycle of Adults, Sleep Drive and Alerting Signal (click to enlarge)

In the morning, it is the Alerting Signal that wakes an adult and drives their energy during the day, even though Sleep Drive is building. In the afternoon, at around 2-4pm most adults experience a Circadian Dip in the Alerting Signal. In some cultures, this is the siesta period, when adults take a short nap to refresh themselves. In other cultures, adults will turn to caffeine or a snack to help wake them up.

About caffeine: caffeine doesn’t actually wake you up. Instead, it blocks the brain’s receptors for Sleep Drive. The Sleep Drive is still there, but your body cannot perceive that you need sleep. The average cup of coffee has about 95mg of caffeine. The half-life of caffeine is 6-hours, so the cup of coffee that you drink at 3pm means that you will still have about 45mg of caffeine in your body at 9pm, when you should be falling asleep. If you drink caffeine, restrict it to mornings and avoid it in the afternoons or it may interfere with your sleep.

In the evening, around 6-8pm, most adults have a surge in Alerting Signal that gives them a Second Wind. During this period, it would be nearly impossible for adults to fall asleep. Although the Sleep Drive is very high, the Alerting Signal is much higher. As bedtime approaches, the Alerting Signal abruptly falls off, and the adult feels sleepy. The fall in the Alerting Signal allows the Sleep Drive to finally take over. Throughout the night, the Sleep Drive continues to fall, as toxins are cleared from the brain.

At about 4am in the morning, adults reach the lower point in their Alerting Signal: this is called the Sleep Nadir.  Sleep Drive is down (but not gone) and the Alerting Signal is very low. This combination makes the most difficult time for adults to wake up.  During the last two hours of sleep, adults experience more R.E.M. sleep than at any other time during the night. R.E.M. sleep is when dreams occur and is considered to be vital to learning and memory. In the morning, the Alerting Signal wakes adults again after about 8 hours of sleep.

What is the Sleep-Wake Cycle of Teens?

Puberty brings a change to sleep for children. Adolescents experience a phase-shift of about 2 hours to their Alerting Signal. This phase-shift means that their Alerting Signal is active about 2 hours later than an adults’ Alerting Signal. This doesn’t happen all at once. On average, an adolescents’ Alerting Signal moves about 10-15 minutes later each year, throughout puberty. So, while a 13 year old might have a shift of 15 minutes, a 18 year old high school senior would have a shift of 2 hours in the Alerting Signal. The chart below shows a 2-hour shift in a teen’s Alerting Signal.

The Sleep-Wake Cycle of Adults vs. The Sleep-Wake Cycle of Teens (Alerting Signal, Sleep Drive, Circadian Dip, Second Wind, Circadian Nadir), Start School Later Masconomet
The Sleep-Wake Cycle of Adults vs. The Sleep-Wake Cycle of Teens: Alerting Signal (click to enlarge)

Teens also need more sleep than adults. The National Sleep Foundation recommends 8-10 hours of sleep for teens, but the average teen needs 9.25 hours of sleep. This is because teen brains are growing and developing and sleep is an essential part of brain health and brain development. An adult is normally sleepy at about 10pm. The phase shift would imply that teens won’t be sleepy until midnight, but because they need an extra hour of sleep, most teens get sleepy at about 11pm.

As with adults, some teens are owls (naturally sleeping from midnight-9am) and others are larks (naturally sleeping from 10pm-7am), but most teens need sleep from about 11pm-8am to feel healthy and rested.

How does school start time interfere with teen sleep?

For adults, the Sleep Nadir (the period when Sleep Drive is low but the Alerting Signal has bottomed out, it’s almost impossible to wake up) is about 4am. For teens, the Sleep Nadir  is phase shifted forward 2 hours, to about 6am. Teens are just as tired at 6am as adults are at 4am, and yet that is exactly the time that most teens are forced to wake up and get ready for school. Their Alerting Signal has not started yet, so they are groggy and would find it easy to fall back to sleep in a matter of minutes. Teens aren’t being difficult when it’s time to go to school: they are deeply tired. Any adult who has had to wake at 4am to go to the airport is familiar with the feeling – now imagine repeating that day, after day, but instead of going to the airport you’ll be heading to 7 hours of classes.

Waking early also interrupts the last two hours of sleep, when teens experience more R.E.M. sleep than at any other time during the night. R.E.M. sleep is when dreams occur and is considered to be vital to learning and memory. In other words, we are not just depriving teens of sleep, but we are robbing them of those crucial R.E.M.-filled hours of sleep – the most vital sleep for their success as students.

The Sleep-Wake Cycle of Adults vs. The Sleep-Wake Cycle of Teens (Alerting Signal, Sleep Drive, Circadian Dip, Second Wind, Circadian Nadir), Start School Later Masconomet
The Sleep-Wake Cycle of Adults vs. The Sleep-Wake Cycle of Teens: Alerting Signal, Sleep Drive, Circadian Dip, Second Wind, Circadian Nadir (click to enlarge)

By around 9am, the teens’ Alerting Signal has started up, and they start to feel a bit more normal. Like adults, teens have a Circadian Dip in the afternoon, though theirs is about 2 hours later – around 4 or 5pm. This is when a 15 minute nap would help them feel more refreshed. Unfortunately, a nap doesn’t make up for the lost sleep or decrease their Sleep Drive, so it’s really a short term solution. A longer nap is no better – it would throw off their Alerting Signal and make it more difficult to fall asleep at bedtime, thus making everything worse.

As the evening goes on, teens feel their most alert from about 8-10pm, experiencing their Second Wind at least 2 hours later than adults do. In order to wake at 6am, teens would need to fall asleep at 9pm, right in the middle of their Second Wind. Teens are no more tired at 9pm than adults are at 7pm, even though their Sleep Drive is very high.

Finally, at about 11pm the Alerting Signal falls and teens are able to sleep. However when they wake again at 6am, they have not received enough sleep to clear their Sleep Drive. This means that they start their day with some Sleep Drive already in place, and it’s harder for the Alerting Signal to overcome the Sleep Drive. Each day, the Sleep Drive builds and teens fall further and further behind.

Teens cannot catch up on sleep on the weekend. Sleeping in more than 1 hour on the weekend has the effect of adjusting the Alerting Signal to be an hour later in the day. Researchers refer to this as Social JetLag. That means that on Sunday night, the teen will not be tired until midnight or even 1am, and as a result on Monday morning, he or she will fall even further behind.

What are the effects of chronic sleep deprivation?

Anyone who has had a newborn is familiar with the feeling: confusion, exhaustion, an inability to concentrate, difficulty remembering things, greater risk of accidents and injury, moodiness, sadness, anxiety, and depression. This is how we send our teens to school every day. We have created arbitrary school hours that make it impossible for teens to fall asleep and wake up naturally.

School starts at Masconomet at 7:35am and the first buses pick up our children at 6:25am. To get enough sleep, our teens would need to be falling asleep at 9pm – or earlier. Given teen biology, that is just not going to happen.

What if we could change the school hours to better reflect teen’s natural sleep habits? What if teens were able to fall asleep and wake up when it felt natural and right for their bodies? What if they could go to school every day well rested and ready to learn?  Why do we insist on forcing them to fit into a schedule that is unnatural and harmful to their health? Yes, parents and children need to set healthy bedtimes, but it is up to schools to set healthy start times that respect our teen’s different biological needs.

It’s time to Start School Later.

Sources for this document are taking from Dr. Judith Owen’s presentation to Greenwich, CT, the National Sleep Foundation, and the American Academy of Pediatrics.