Sleep is the most active “inactive” part of your life. Although your muscles are relaxed and still (sleep walkers excluded!), the rest of your body is working hard to rebuild your foundation for the next day. And doing so correctly requires a certain architecture. Sleep architecture refers to the three primary phases of sleep essential for this process; here's how they come together to build you a strong tomorrow.
Across a single eight-hour night’s sleep, you go through about five sleep cycles, each lasting for approximately 90 minutes (an example is outlined above). Each cycle contains three phases: light sleep (stages 1 & 2 in the above diagram), deep or slow wave sleep (stage 3), and Rapid Eye Movement Sleep (REM). PS: The first two phases are therefore referred to as NREM – Non-Rapid Eye Movement sleep. Now, let’s explore each of these phases individually.
Light Sleep (NREM Stage 1 and 2)
As you fall asleep, the electrical activity in your brain changes. When you're awake, signals are constantly being transmitted across your synapses for things ranging from conscious thoughts to the perception of sounds. In fact, looking at the brainwaves of an awake person, you would see multiple random, jagged, and shallow peaks and valleys, indicating constant stimulation and processing.
However, as you transition into light sleep, brainwaves become more regular and grow in amplitude – meaning they become deeper – as the conscious parts of your brain slow down. This is when sleep starts to be restorative; once you shut off resources to certain parts of your brain, it reallocates these resources to the parts that are still active, allowing them to normalize and repair. In fact, even a quick nap (which, by definition, only includes light sleep) can increase alertness and cognition for up to three hours.1,2,3
However, light sleep phases primarily serve as a transition period to deeper ones. So if you're someone who has trouble falling asleep, focus on healthy bedtime habits to help you get there. This includes avoiding coffee and snacks in the evening, exercising during the day rather than after dusk, and keeping your room cool, dry, and dark.
Optimal cortisol levels are also key for a healthy night's sleep; while cortisol is indeed essential for multiple nervous system functions, it can also keep you awake.4,5 In fact, An increase as small as 1 ug/dL can increase your time spent tossing and turning by over 20 minutes in one night.10 Over the course of a single month, that’s the equivalent of losing nearly one-and-half full nights of sleep. So bottom line: so track your changes in this biomarker over time!
Deep or Slow Wave Sleep (NREM Stage 3)
Deep or slow wave sleep is, as the name suggests, characterized by long, deep brainwaves. It's during this stage that your senses and voluntary movements are completely stopped. However, your brain is far from inactive; studies suggest that slow wave sleep is essential for solidifying new memories made during the previous day.6,7,8,9
Slow wave sleep reactivates the memories made while you were awake and transfers them to long-term storage.6 More specifically, this phase is critical for storing declarative memories – ones that require conscious thought like that of facts or experiences of events. 7,8 Procedural memories (unconscious ones like the ability to ride a bike) are largely unchanged by slow wave sleep.
So, if your memory has been foggy lately (or for as long as you can remember), it may be due to inadequate time spent in slow wave sleep. And how do we inch our way into slow-wave territory? Well, you might not be surprised to learn that the answer again lies in your cortisol levels. Just like it delays the onset of light sleep, high cortisol levels can prevent you from entering this next critical phase.4
But be aware: cortisol levels can also be too low, resulting in fatigue. How does sleep affect this? Well, cortisol levels follow a circadian rhythm, typically peaking between 6:00 and 8:00 AM – your body’s natural waking time. So if you routinely stay up late or sleep in to make up for night owl habits, your cortisol levels may dip below optimal as your body tries to encourage more sleep hours. And long term, misalignment from natural sleep patterns (greater than 1-3 months) can reduce cortisol by as much as 40 percent.
If you’re struggling to hit your sleeping rhythm, set yourself up for a natural sleep/wake cycle. Set yourself an alarm both for bedtime and waking up – restricting rest to the same hours each day can help spur your body into the patterns it craves and keep cortisol levels optimal.
Interestingly enough, our brain during REM resembles that of an awake one: lots of random, shallow brain waves.
Dreaming mostly happens during the REM phase (although dreaming can take place during NREM), which may explain why REM brainwaves look similar to awake ones – dreams can be very stimulating. The key difference? REM thought patterns tend to be driven by emotion more than thought. 11 This is why dreams tend to be vivid and, well, dream-like, but can often be nonsensical. So it makes sense that, while slow wave sleep solidifies logical memories, REM solidifies emotional memories and softens the blow of negative ones.12,13,14,15,16
Now, while earlier sleep phases are associated with cortisol, REM sleep is associated with its counterpart: testosterone. Unlike cortisol’s circadian rhythm, though, testosterone starts its rise when you fall asleep, peaking during your first REM stage of the night.17 And much like cortisol, an abnormal sleep cycle can significantly impact your T levels. In fact, peak T levels can drop by 10 percent in a single night, and 41 percent over the course of a week.18,19,20
Finally, REM sleep is the most heavily impacted by loss of sleep – it’s the first phase to be sacrificed if you don’t sleep long or deep enough; a full REM phase (about 30 minutes) can be lost each night.4,5 This loss of sleep is also associated with distortions in glucose, lipid, and iron levels; just a few nights of sleep interruption or restriction can decrease insulin sensitivity by 25 percent, which can lead to memory impairment and accelerated aging.21 Serum iron and hemoglobin may also decrease, while LDL and triglycerides increase.22,23,24,25
Sleep On It
Each phase of our sleep cycles are affected by different factors in our lives and have very different impacts on our overall wellbeing. Getting a good night's sleep means more than a simple 8 hours – it means properly cycling through our sleep architecture. By making positive changes for each of the phases, you can get the most bang for your bedtime. Looks like “you snooze, you lose” isn’t true after all – better sleep means better health!
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 Martin-Gill, Christian, et al. "Effects of napping during shift work on sleepiness and performance in Emergency Medical Services personnel and similar shift workers: a systematic review and meta-analysis." Prehospital Emergency Care 22.sup1 (2018): 47-57.
 Milner, Catherine E., and Kimberly A. Cote. "Benefits of napping in healthy adults: impact of nap length, time of day, age, and experience with napping." Journal of Sleep Research 18.2 (2009): 272-281.
 Lovato, Nicole, and Leon Lack. "The effects of napping on cognitive functioning." Progress in Brain Research. Vol. 185. Elsevier, 2010. 155-166.
 Friess, Elisabeth, et al. "Acute cortisol administration promotes sleep intensity in man." Neuropsychopharmacology 29.3 (2004): 598.
 Vgontzas, Alexandros N., et al. "Impaired nighttime sleep in healthy old versus young adults is associated with elevated plasma interleukin-6 and cortisol levels: physiologic and therapeutic implications." The Journal of Clinical Endocrinology & Metabolism 88.5 (2003): 2087-2095.
 Walker, Matthew P. "The role of slow wave sleep in memory processing." Journal of Clinical Sleep Medicine: JCSM: official publication of the American Academy of Sleep Medicine 5.2 Suppl (2009): S20.
 Maquet, Pierre. "The role of sleep in learning and memory." Science 294.5544 (2001): 1048-1052.
 Rauchs, Geraldine, et al. "The relationships between memory systems and sleep stages." Journal of Sleep Research 14.2 (2005): 123-140.
 Born, Jan. "Slow-wave sleep and the consolidation of long-term memory." The World Journal of Biological Psychiatry 11.sup1 (2010): 16-21.
 Wright Jr, Kenneth P., et al. "Influence of sleep deprivation and circadian misalignment on cortisol, inflammatory markers, and cytokine balance." Brain, behavior, and Immunity 47 (2015): 24-34.
 Mirick, Dana K., et al. "Night shift work and levels of 6-sulfatoxymelatonin and cortisol in men." Cancer Epidemiology and Prevention Biomarkers (2013).
 Hobson, J. Allan, Edward F. Pace-Schott, and Robert Stickgold. "Dreaming and the brain: toward a cognitive neuroscience of conscious states." Behavioral and Brain Sciences 23.6 (2000): 793-842.
 Rihm, Julia S., and Björn Rasch. "Replay of conditioned stimuli during late REM and stage N2 sleep influences affective tone rather than emotional memory strength." Neurobiology of learning and memory 122 (2015): 142-151.
 Genzel, Lisa, et al. "The role of rapid eye movement sleep for amygdala-related memory processing." Neurobiology of learning and memory 122 (2015): 110-121.
 Landmann, Nina, et al. "REM sleep and memory reorganization: potential relevance for psychiatry and psychotherapy." Neurobiology of learning and memory 122 (2015): 28-40.
 Cairney, Scott A., et al. "Complementary roles of slow-wave sleep and rapid eye movement sleep in emotional memory consolidation." Cerebral Cortex 25.6 (2014): 1565-1575.
 Luboshitzsky, R., et al. "Relationship between rapid eye movement sleep and testosterone secretion in normal men." Journal of Andrology 20.6 (1999): 731-737.
 Luboshitzky, R., et al. "Disruption of the nocturnal testosterone rhythm by sleep fragmentation in normal men." The Journal of Clinical Endocrinology & Metabolism 86.3 (2001): 1134-1139.
 Arnal, P. J., et al. "Effect of sleep extension on the subsequent testosterone, cortisol and prolactin responses to total sleep deprivation and recovery." Journal of Neuroendocrinology 28.2 (2016).
 Schmid, Sebastian M., et al. "Sleep timing may modulate the effect of sleep loss on testosterone." Clinical Endocrinology 77.5 (2012): 749-754.
 Stamatakis, Katherine A., and Naresh M. Punjabi. "Effects of sleep fragmentation on glucose metabolism in normal subjects." Chest 137.1 (2010): 95-101.
 Kuhn, E., and V. Brodan. "Changes in the circadian rhythm of serum iron induced by a 5-day sleep deprivation." European journal of applied physiology and occupational physiology 49.2 (1982): 215-222.
 Ståhle, Lars, et al. "Effects of food or sleep deprivation during civilian survival training on clinical chemistry variables." Wilderness & environmental medicine 24.2 (2013): 146-152.
 Kerkhofs, Myriam, et al. "Sleep restriction increases blood neutrophils, total cholesterol and low density lipoprotein cholesterol in postmenopausal women: a preliminary study." Maturitas 56.2 (2007): 212-215.
 Ollila, Hanna Maria, et al. "TRIB1 constitutes a molecular link between regulation of sleep and lipid metabolism in humans." Translational psychiatry 2.3 (2012): e97.