Conventional wisdom suggests that the longer you go without sleep, the more tired you should be. But if you’ve ever pulled an all-nighter, you’ve probably experienced the opposite phenomenon: At 4 a.m., you can barely keep your eyes open. Then, it’s 8 a.m. and you’re wide-awake with seemingly normal energy.
Why do you feel less sleepy as time goes on instead of more?
Scientists have long suspected it’s because our brains follow a natural circadian rhythm that tells us to wake up when the sun comes up. Now, a new study provides some of the strongest evidence yet that this theory is correct. But in the course of confirming this finding, researchers discovered something totally surprising.
The brain has multiple ‘clocks’
MRI scans of brain activity in a group of adults who stayed awake overnight revealed that sleepiness was related to two factors: the amount of time each person had stayed awake, and the circadian rhythm, an internal clock that tells our bodies to feel awake in the mornings when the sun comes up and tired during dark morning hours and at the end of the day.
The scientists’ unprecedented discovery was that not all regions of the brain were affected by these two factors in the same way at the same time. In other words, some parts of your brain take stronger cues from the fact that you spent the night trying to keep your eyes open, and other parts of your brain care more about the fact that the clock says 8 a.m. And how strongly different parts of your brain react to either cue changes throughout the day.
Yep, there’s a lot going on upstairs!
“This is the first time there is evidence that we have multiple brain clocks,” study co-author Pierre Maquet, a sleep researcher at Belgium’s University of Liège, told The Huffington Post.
Plenty of studies that have observed human behavior (not to mention our own personal experiences) tell us that part of what drives sleepiness is how long we’ve been awake. Scientists call it our sleep drive. Think of it like an hourglass: The longer we’re awake, the more we feel the urge to sleep. And the longer we’re asleep, the more we feel the urge to wake up.
But research has established that our bodies are also governed by the natural circadian rhythm that runs on a roughly 24-hour cycle. It dips between approximately 2 and 4 a.m. and 1 and 3 p.m., causing us to feel sleepiest at those times.
For this new study, the researchers used MRI scans to monitor brain activity in 33 adults who stayed awake for 42 hours straight, as well as after a 12-hour period of recovery sleep. The scans showed that blood flow, which indicates how active the brain is at a given point, actually fluctuates according to circadian rhythm in some areas and according to sleep drive in other areas.
While awake the individuals completed a series of tests that measured their reaction time in order to show how it changed the longer they had not slept.
“For 16 to 18 hours [approximately], your cognitive performance stays fairly constant,”Maquet said. “And if you stay awake longer during the night your performance will decrease, reaching its minimum around 4 a.m. to 6 a.m.
“[The MRI scans showed that] the various brain areas that are engaged in a given task are not exactly in the same circadian phase, suggesting there is a local modulation of the brain circadian rhythmicity,” Maquet said.
Basically, it’s as if different parts of your brain have their own time zone. Some parts of the brain closely followed the expected circadian schedule, while other parts were more affected by the accumulation of sleep debt.
This is an important finding because it changes scientists’ understanding of how spending more hours awake affects the brain and specific brain functions, Maquet said.
More evidence shift work and jet lag mess with our heads
Maquet said the new research may recast how we think about the effects of shift work, jet lag, sleep disorders and even aging on our bodies and on our cognitive functioning. This study lends more evidence that circadian misalignment (due to shift work, for example) can deteriorate sleep quality, and thus interfere with other parts of our health, Maquet explained.
What’s more, the new research may lead to new ways to better diagnose sleep disorders, Charles Czeisler, chief of the division of sleep and circadian disorders at Brigham and Women’s Hospital, told HuffPost. Sleep disorders can be difficult to discern from other disorders that share the same symptoms, such as attention deficit hyperactivity disorder, he said.
“[The study] opens up a new era in the quantitative assessment of sleep-wake neurobiology,” Czeisler wrote in an editorial that was published alongside the new study in the journal Science.
The research suggests there is a way to actually measure the deterioration in brain activity that’s associated with longer periods of being awake.
“We might be able to investigate now if in certain conditions ― like sleep apnea ― is the brain more sluggish? Are certain areas of the brain more affected?” Czeisler said.
Unfortunately for procrastinators everywhere, the new research sheds no light on the how to stay up all night and not suffer the consequences the next day.
Sarah DiGiulio is The Huffington Post’s sleep reporter. You can contact her at firstname.lastname@example.org.