What is consciousness? “Consciousness is what allows you to think, remember, and feel things.” It includes awareness of yourself. Descartes’ famous line. “I think, therefore I am,” declared his consciousness.

Conscious thinking means our brains, our minds, are sensing, observing, memorizing, recalling, decoding, analyzing, calculating, interrelating, cross-referencing, rearranging, expanding, generalizing, communicating, and even creating. Those coordinated operations, part of cognition, require real work.

After all that brain work, it should be time for a rest, right? Nope. When a supermarket closes, the workers don’t just switch off the lights and go home. Overnight the workers clean, restock, organize, repair, and get the store ready for the next day. It’s the same for the brain. Lie down, close your eyes, lose consciousness, and the brain undertakes the heavy lifting that sleep demands.

Revealing the how and why of sleep, Matthew Walker, professor of neuroscience and psychology at UC Berkeley, has written Why We Sleep: Unlocking the Power of Sleep and Dreams. Sleep is not just an annoying state of rest that wastes valuable time. As Walker describes:

Sleep is not the absence of wakefulness. It is far more than that. [O]ur nighttime sleep is an exquisitely complex, metabolically active, and deliberately ordered series of unique stages.

Drawing from observed behaviors, self-reports, three-dimensional brain scans, and experiments upon humans and animals, scientists have uncovered much about what goes on during the brain’s night shift.

In-Built Brain Operation Pattern   

Start with the approximately 24-hour cycle that regulates our brain every day. Adults experience this predictable and repeating pattern, called the circadian rhythm, as a period of steadily increasing energy, body temperature and wakefulness followed by a period of steady decrease of all three and then back again. If not interrupted or distracted from the cycle, adults would experience 15 hours awake followed by 9 hours of sleep, day after day. Our eyes detect the sunlight appearing in the 24-hour day and transmit signals consistent to the back of the brain to keep the cycle synchronized.

The Nighttime Switcheroo

What triggers the shift from the awake state to the sleeping state? A chemical called adenosine builds up slowly in the brain while we’re awake. At a certain level of adenosine concentration, Walker explains, “an irresistible urge for slumber will take hold.” This point usually occurs after we have been awake for between 12 and 16 hours, coinciding with when the circadian rhythm has independently been dipping our energy levels also.

Another brain chemical, melatonin, triggers the brain to sleep. Shortly after sundown, the brain instructs the pineal gland to release melatonin into the blood stream. Melatonin does not cause sleep, but it does send the chemical message that sleeping time is near.

Unless interrupted or distracted, we will and should sleep for 8 to 9 hours. During that period, the brain’s biochemical functions steadily reduce the adenosine level. It takes about 8 hours to empty out nearly all the adenosine — and at that point, the circadian rhythm is starting to ascend back up the energy cycle. These coordinated biochemical cycles follow a predefined plan, so we sleep and then naturally awaken to a new day.

Mind-Blowing Sleep Functions

Once we’re asleep, the brain and body don’t actually rest. True, the body tends to relax the postural muscles, and the person stops communicating and responding to most outside sensory inputs. But the brain shifts into extremely active phases of operation.

While we’re awake, we experience consciousness and cognition. Those functions pause, giving way to sleep, which involves two main categories. One is REM, named for the Rapid Eye Movement that occurs in humans during this sleep phase. The other category is Non-REM sleep (NREM). At least three stages of different brain electrical activities occur within NREM. All mammals and birds have been seen to experience both NREM and REM sleep phases.

Humans experience a 90-minute cycle of sleep passing first through the NREM and then the REM phases. As Why We Sleep details, the two major phases carry out extraordinary amounts of work.

During NREM Sleep the Brain:

• Refreshes memories
• Selects needed and helpful memories for retention
• Identifies unneeded and painful memories to be forgotten     
• Transfers fact-based memory information from temporary storage to long-term storage
• Clears short-term memory storage regions for future use
• Replenishes learning ability

During REM Sleep the Brain:            

• Sharply increases activity in brain regions for visual, motor, memories of experiences, and especially emotions
• Deactivates edges of prefrontal cortex, switching off logical thought in the control region
• Solidifies language acquisition
• Recalibrates the ability to determine the meaning of facial expressions and body language
• Works toward resolving emotional reactions and states, reducing hypersensitivity
• Works to solve problems
• Engages in creativity, e.g., by associating stored memories not obviously related

Overall, the Brain During Sleep: 

• Protects newly acquired information
• Consolidates memory against forgetting what is important
• Builds “skill memory,” continuing to improve the “skill memory” without actual practice
• Restores and enhances the ability to focus and pay attention while awake
• Restores and enhances the speed and effectiveness of reactions to challenges and dangers        
• Balances emotional rationality and stability

Sleep Restores Both Brain and Body

Typically, we think of humans as possessing a “mind,” and we may observe some animals as seeming to have a mind as well. People debate whether the “mind” is entirely part of the brain (monism) or is separate in some ways from the physical brain (dualism). It appears the sleep function is mostly if not entirely a physical brain operation, however. Not only humans but also animals such as reptiles, birds, and mammals experience deep sleep. Even fruit flies sleep.

The electroencephalograph (EEG) device can detect brain wave patterns that identify sleep. EEG studies and other techniques have confirmed sleep patterns in all these creatures. The brain’s sleep activity presents immense challenges to our understanding of physical brains, far beyond the already unfathomable complexity of animal and human cognition and consciousness.

Knowing the brain’s night shift operations nevertheless helps us realize how sleep deprivation and sleep interruptions such as occur with sleep apnea are not mere annoyances but actually damage a whole array of functions that work best when they run in their designed sequences for sufficient time.

Additionally, as reported in the Nature and Science of Sleep journal, “sleep is vital to most major physiologic processes,” and “sleep disruption has vast potential for adverse short- and long-term health consequences,” such as: “heightened stress response; pain; depression; anxiety; and cognition, memory, and performance deficits.” Long term effects of disrupted sleep include hypertension, cardiovascular disease, weight gain, metabolic syndrome, and type 2 diabetes.

Bottom line: Sleep is not just rest but is a life phase absolutely needed to support mind, brain, and body operations. The brain, like the heart and other subsystems, never rests.