A Brief Review of Sleep: From A to Zzz…

We’ve focused quite a bit on physical activity so far in “What Does the Science Say?” but it’s time to take a step back and discuss what we often consider a passive activity but is really one of the most important parts of our day (well, night typically): Sleep.

Now, I could talk (or write) about the science of sleep extensively--the numerous benefits, the dangers of inadequate sleep, or how to work on improving your sleep quality. Unfortunately, Episode 3 doesn’t have enough space for that, so below is an overview of sleep, with a few of my favorite studies,  eye-opening stats, and some of the data gathered by our wearable devices.

The Importance of Sleep

Sleep is essential to our day-to-day functioning (and if you have any doubts, I strongly discourage you from finding out the hard way… because sleep deprivation is no joke). 

In one study, participants took part in a “28-hour cycle where each new ‘day’ created a  4 hour shift in participants’ circadian rhythms. When participants were examined while operating at a 12-hour flip from their typical schedule (i.e. night had become day for them), scientists recorded the following: higher blood pressure, lower levels of the hormone leptin (which helps you stop eating), and remarkably, 6% higher blood sugars despite 22% higher insulin secretion (which helps you lower your blood sugar). In fact, almost half of the participants at this step, showed glucose patterns similar to individuals with prediabetes[1].

Sleep is a time when the body can accomplish a lot of the general upkeep needed to perform well in our day-to-day. Through complex (and not yet entirely understood) mechanisms, sleep helps maintain our immunity, regulate our emotions through neurotransmitter responses, maintain our metabolism, and alter new synaptic pathways in the brain[2-4]. While sleep is a rejuvenating and restful experience, it is a relatively active process for the body, affecting almost every aspect of our wellbeing. As such, it is important to ensure we are getting appropriate sleep in order to achieve these goals.

How to maintain healthy sleep

One metric of healthy sleep (which you can monitor in Athlytic) is sleep consistency. Going to bed at similar times each day and maintaining a consistent circadian rhythm helps to maintain sleep quality. In fact, several studies have suggested that sleep consistency (similar timing for when you go to bed and wake up) may be more important in the long term than sleep duration alone[5]. So be sure to check the Athlytic advanced sleep stats to monitor your consistency.

Sleep debt is another important metric to consider. Increasingly, as we aim to improve productivity and performance some of us ignore our body’s calls for sleep. Yet if we are not able to get enough sleep, we will not be able to adequately keep up with all of the sleep functions mentioned above (immunity, emotion regulation, etc.), and our body will start to suffer the consequences. Inadequate sleep time has been consistently linked to worsened cognition, metabolic health, and cardiovascular disease[6]. Indeed, an increased sleep debt increases our sympathetic tone/responses, which will influence (decrease) our heart rate variability and recovery. (Find more info about this on Episode One!) Unfortunately, napping and weekend ‘catch-ups’ do not fully compensate for chronic sleep dept – so it’s important to take a preventative approach here[7].

So what do my numbers mean?
Sleep science is still a vastly growing field. Only recently did technology progress to the point where we have been able to get reliable, medical grade sleep information at home, without needing multitudes of wires and sensors distracting us as we “sleep” in an unfamiliar room on an old bed (i.e. the traditional sleep study).

At Athlytic, we rely on Apple Watch data, which can monitor several important factors like oxygenation, heart rate, and even temperature in addition to estimating our sleep stages.

Can you back up and tell me more about Sleep Stages?

Our typical sleep is broken up into several stages, ranging from light to deep sleep: N1, N2, N3, and REM. Higher numbers correlate with deeper sleep.  Athlytic and Apple watch data is categorized as Core (N1&2), REM, and Deep (N3) sleep. 

Throughout the night, we cycle through these various stages repeatedly. Each time the REM cycle gets slightly longer (initially ten minutes or so, but ending at up to an hour), and the deep sleep (N3) gets progressively shorter. A complete cycle through the stages takes roughly 2 hours or so.

In our core sleep, body temperature and heart rate begins to drop. This stage appears to be the most important for cognitive building, memory formation, etc. As we continue to progress to deeper sleep, we start to work on tissue and muscle repair and rebuilding. If awakened from deep (N3) sleep, we generally experience brain fog, fatigue, etc. which is why this is the hardest stage to awaken someone from. (This is also the stage where sleepwalking and night terrors tend to occur.)

REM sleep is a much more active process on the brain, though the skeletal muscles are in a semi-paralyzed state. Brain waves are similar here as they would be when awake and this leads to dreaming. Generally, this stage is what will result in spontaneous (and rested) awakening in the morning[8].

So what is my goal, again?

Fortunately, Apple has provided validation studies for its sleep tracking, and the results are very promising. While technology continues to progress and evolve every year, data from Apple Watch sleep estimation appears reliable and consistent. In internal studies, the most common error is attributing some Deep Sleep (N3) to Core Sleep (N1&2). Because of this, deep sleep is often 10-15% of sleep time, REM sleep represent about 25% of sleep time, and Core sleep makes up the other approximately two-thirds.

Conclusions

Sleep is a complex process, and getting the optimal quantity and quality of sleep is much easier said than done. Take advantage of the Sleep Performance metrics in order to monitor your sleep duration, sleep consistency, and sleep debt. Using the tagging feature can help you identify individual triggers that promote healthy sleep and learn to avoid those that disrupt. (I’m looking at you alcohol.)

While there is certainly more to discuss on future episodes, I hope this helped provide some background and context into the importance of sleep monitoring. Stay active, stay healthy, and stay rested.

Sources and further reading

1. Scheer FAJL, Hilton MF, Mantzoros CS, Shea SA. Adverse metabolic and cardiovascular consequences of circadian misalignment. Proc Natl Acad Sci U S A. 2009;106(11):4453-4458. doi:10.1073/pnas.0808180106

2. Garbarino S, Lanteri P, Bragazzi NL, Magnavita N, Scoditti E. Role of sleep deprivation in immune-related disease risk and outcomes. Commun Biol. 2021;4:1304. doi:10.1038/s42003-021-02825-4

3. Koren D, Dumin M, Gozal D. Role of sleep quality in the metabolic syndrome. Diabetes Metab Syndr Obes. 2016;9:281-310. doi:10.2147/DMSO.S95120

4. Vandekerckhove M, Wang Y lin. Emotion, emotion regulation and sleep: An intimate relationship. AIMS Neurosci. 2017;5(1):1-17. doi:10.3934/Neuroscience.2018.1.1

5. Windred DP, Burns AC, Lane JM, et al. Sleep regularity is a stronger predictor of mortality risk than sleep duration: A prospective cohort study. Sleep. 2024;47(1):zsad253. doi:10.1093/sleep/zsad253

6. Spiegel K, Leproult R, Cauter EV. Impact of sleep debt on metabolic and endocrine function. The Lancet. 1999;354(9188):1435-1439. doi:10.1016/S0140-6736(99)01376-8

7. Leger D, Richard JB, Collin O, Sauvet F, Faraut B. Napping and weekend catchup sleep do not fully compensate for high rates of sleep debt and short sleep at a population level (in a representative nationwide sample of 12,637 adults). Sleep Med. 2020;74:278-288. doi:10.1016/j.sleep.2020.05.030

8. Patel AK, Reddy V, Shumway KR, Araujo JF. Physiology, Sleep Stages. In: StatPearls. StatPearls Publishing; 2024. Accessed February 14, 2024. http://www.ncbi.nlm.nih.gov/books/NBK526132/