Brain Scans Reveal Why It Takes So Long to Wake Up in the Morning

"Sleep inertia" is real, and caffeine won't help.

Every morning, people sleepily drag themselves out of bed, wandering through a brain fog that seems to take forever to dissipate. Early risers will deny it exists, but evidence in a new paper in the journal NeuroImage suggests otherwise. The University of California, Berkeley team behind the study also reveal the one way to get through it.

The term for that cognitive fog is “sleep inertia,” but before the current study we’ve never been quite sure why people experience it, says Raphael Vallat, Ph.D., the lead study author and post-doctoral fellow at The University of California, Berkeley. In the paper, he proposes a reason why it exists: Even when the body is awake and moving in the morning, its brain is asleep in some capacity for some time after.

Unsplash / Sabri Tuzcu

“When we wake up from sleep, our brain does not immediately switch from a sleep state to a fully awakened state but rather goes through this transition period called sleep inertia that can last up to 30 minutes,” Vallat tells Inverse. “During this period, the brain progressively switches from sleep to normal wakefulness, and so does our mental/cognitive performance.”

To demonstrate how real this transitional period is, Vallat had 34 participants take 45-minute naps in which they entered two periods of deep sleep known as N2 and N3. (They didn’t, however, enter rapid eye movement (REM) sleep — the deepest type of slumber.) When they woke up, Vallat tested their alertness with two subtraction tests, one five minutes after waking up and another one 25 minutes after waking up.

As anyone who’s experienced brain fog might expect, the subjects tended to make more mistakes right upon awakening — and their brain scans hinted at why.

When we’re awake, the brain oscillates between two different “modes” that occur in two separate circuits: a focused, task-active mode (which we use when reading or being productive) and a non-focused, task-negative mode (which is for mind-wandering). While we’re awake, we switch between these two modes: When the task-active mode is functional, there is usually a decrease in activity in the task-negative circuit.

The brain fog you experience each morning has a name: "sleep inertia."

Unsplash / Annie Spratt

What makes the period of “sleep inertia” different, Vallat says, is that the brain struggles to switch fluidly between circuits.

“So, it’s as if our brain was not really able to switch between these two modes, and as a consequence, we also found that our participants had lower performance during sleep inertia in a mental calculation task,” he says.

Vallat’s results show that during the “sleep inertia” period, the brain slowly regains the ability to switch between these two modes, divided by “functional segregation.” He believes that it takes about 30 minutes to fully achieve this.

Unfortunately, Vallat laments, there’s not much we can do to speed up the wakeup process. Not even a caffeine boost is a true solution.

“There are some results that show that caffeine increases the functional segregation between the task-active and task-negative networks, thus enhancing the brain’s abilities to switch between these two modes,” Vallat says. But it may not actually work fast enough to cut through the sleep inertia.

“First, caffeine takes 30 to 60 minutes to reach its peak level, and we know that sleep inertia usually dissipates in 30 minutes, so even before the caffeine would actually start to have a strong action on your body,” he adds.

Instead of attempting to caffeinate through a period of slow brain functioning, Vallat recommends that perhaps the only real tonic for sleep inertia is time.

“The best thing to do is certainly to wait for a few minutes before making any important decisions or hitting the road, especially if you feel that you have just woken up from a deep slumber,” he recommends.

The first minutes following awakening from sleep are typically marked by reduced vigilance, increased sleepiness and impaired performance, a state referred to as sleep inertia. Although the behavioral aspects of sleep inertia are well documented, its cerebral correlates remain poorly understood. The present study aimed at filling this gap by measuring in 34 participants the changes in behavioral performance (descending subtraction task, DST), EEG spectral power, and resting-state fMRI functional connectivity across three time points: before an early-afternoon 45-min nap, 5 min after awakening from the nap and 25 min after awakening. Our results showed impaired performance at the DST at awakening and an intrusion of sleep-specific features (spectral power and functional connectivity) into wakefulness brain activity, the intensity of which was dependent on the prior sleep duration and depth for the functional connectivity (14 participants awakened from N2 sleep, 20 from N3 sleep). Awakening in N3 (deep) sleep induced the most robust changes and was characterized by a global loss of brain functional segregation between task-positive (dorsal attention, salience, sensorimotor) and task-negative (default mode) networks. Significant correlations were observed notably between the EEG delta power and the functional connectivity between the default and dorsal attention networks, as well as between the percentage of mistake at the DST and the default network functional connectivity. These results highlight (1) significant correlations between EEG and fMRI functional connectivity measures, (2) significant correlations between the behavioral aspect of sleep inertia and measures of the cerebral functioning at awakening (both EEG and fMRI), and (3) the important difference in the cerebral underpinnings of sleep inertia at awakening from N2 and N3 sleep.
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