How Accurate are Smart Watches?
Wearable devices have as much as 20% error when measuring heart rate, and caloric expenditure measurements can be off by as much as 100%. Most wearable fitness devices overestimate total sleep time and underestimate wakefulness after sleep onset.
SMART WATCH ACCURACY KEY TAKEAWAYS
- Sleep is an essential biological process that plays a crucial role in enhancing adaptive capacity.
- The two primary processes that govern our sleep patterns are sleep-wake homeostasis and circadian rhythm.
- Sleep can improve recovery, boost the immune system, enhance physical and mental resilience, and improve performance outcomes.
- To get quality sleep, it's important to focus on duration (7-9 hours/night), timing (early bedtime & early wake time), consistency (same schedule during week & weekend), light exposure, temperature, and avoiding caffeine late in the day or alcohol before bedtime.
- Regular quality sleep helps restore the body, detoxify the brain, improve emotional regulation, enhance recovery from exercise, and balance hormones.
Sleep is an essential biological process required for humans to grow, adapt, and thrive under stress. It is a restorative function consistently regarded as the most efficacious strategy for improving recovery, boosting the immune system, and enhancing physical and mental resilience and performance outcomes. But what exactly is sleep, and how can you use it to build adaptive capacity?
Understanding Why We Sleep
Sleep is a complex physiological process that we still don’t fully understand. However, research indicates that two primary processes govern our sleep patterns:
- Sleep-wake homeostasis
- Circadian rhythm.
Each serves a unique role in our sleep cycle, but together they create a balanced and healthy sleep schedule.
First, let's talk about sleep-wake homeostasis. Think of this as a kind of 'sleep meter' in your body. The longer you're awake and active, the more your body craves sleep. Researchers often refer to this as sleep pressure. The 'sleep meter' fills up, making you feel more tired, and it only empties when you sleep. The fundamental molecule involved in this process is adenosine, which accumulates in your brain during your wakeful hours and decreases when you're asleep.
But it's not all about how long you've been awake or asleep. That's where the circadian rhythm comes into play. Imagine your body has an internal 24-hour clock, and it's ticking away in your brain, dictating periods of sleepiness and alertness. This natural rhythm is influenced heavily by environmental light levels, keeping you alert during the daylight hours and nudging you towards sleep when it's dark.
Now, you might be wondering who's the chief orchestrator of this biological symphony? Enter the suprachiasmatic nucleus (SCN), a small part of the brain located in the hypothalamus, often referred to as the "circadian pacemaker." The SCN controls the production of melatonin, the sleepy hormone (1).
The SCN communicates with our optic nerves, which relay light information from our eyes to our brain. As the sun sets and light levels decrease, the SCN receives the cue to ramp up melatonin production, making us feel sleepy. Conversely, as dawn breaks and the light levels increase, the SCN dials back on melatonin production and increases core temperature and cortisol, signaling our bodies to wake up and be alert.
So, in a nutshell, our circadian clock is our body's internal scheduler, determining when we should be up and about or tucked up in bed. Guided by light signals, our suprachiasmatic nucleus acts as the manager of this clock, making us feel awake or tired at the appropriate time.
The Role of Sleep in Adaptive Capacity
Sleep plays a crucial role in enhancing our adaptive capacity in several ways:
Turning Down Stress
Sleep is the primary restorative period for our tissues, immune, and endocrine systems. During sleep, our stress systems are down-regulated, including the hypothalamic pituitary adrenal axis (HPA axis) and sympathetic nervous system. During wakefulness, these systems are very active, which is great, as they empower your brain and body to do complex tasks like problem-solving, exercising, and engaging in meaningful conversations.
However, these systems suppress your immune system. Sleep turns off these stress systems, enabling your immune system to kick into high gear. During sleep, our body initiates adaptive immune responses crucial for immunological memory — the ability to recognize and fight off invaders such as bacteria and pathogens that ultimately make us ill.
During sleep, one of the key hormones that is decreased is cortisol. Although a consistent and significant cortisol spike in the morning is important for improving energy and alertness, chronically elevated cortisol levels can increase stress reactivity and suppress the immune system.
During slow wave or deep sleep, the pituitary gland releases growth hormone which is vital for tissue regeneration, body composition, and healthy brain function. In addition, for men, a significant amount of testosterone is released during REM sleep. Sleep fragmentation and sleep apnea are associated with reduced testosterone levels, and the relationship is so strong that you can predict morning testosterone levels in older men by measuring total sleep time.
Men lose about 1% of their natural testosterone production per year after age 30. Reducing sleep to just 5 hours per night for one week will age a man by a decade in terms of testosterone. This can be devastating, as testosterone impacts libido, bone mass, fat distribution, and the ability to synthesize muscle protein.
Sleep also impacts your ability to maintain a healthy body composition. Poor sleep patterns can lead to an imbalance in your hunger hormones, leptin and ghrelin. Leptin decreases your appetite, while ghrelin increases it.
Research demonstrates that when you experience partial sleep deprivation (less than 7 hours/night), the average person eats an additional 385 calories. Over time this can lead to significant weight gain and an increased risk for diabetes and other adverse health conditions that impair adaptive capacity.
Detoxification of the Brain
The brain undergoes profound detoxification during sleep. The glymphatic system essentially detoxifies the brain, helping to remove harmful metabolic waste products, such as amyloid-beta, a protein that can build up in the brain and can contribute to the development of neurodegenerative diseases like Alzheimer’s disease.
The glymphatic system is primarily active during sleep. Glymphatic clearance is almost 10 times more active during sleep as structural changes occur in the brain along with an increase in the flow of cerebrospinal fluid into the brain, which helps flush out toxic waste.
Glymphatic clearance during sleep may explain why you feel revitalized and clear-headed after a restful night of sleep but may experience brain fog or have difficulty solving complex tasks after a poor night of sleep. Overall, sleep significantly enhances the functioning of the glymphatic system, making it an essential factor in maintaining brain health and potentially preventing neurodegenerative diseases.
The emotional centers of the brain, particularly the amygdala and the prefrontal cortex, are greatly affected by sleep. These brain regions are responsible for emotional regulation, decision-making, and social interaction. Sleep deprivation can heighten the amygdala's reactivity to negative stimuli, leading to more intense emotional reactions and poorer stress management. Adequate sleep can help to maintain balance between these brain regions and improve emotional resilience.
Lack of sleep can impact cognitive functions such as concentration, memory, and decision-making capabilities. These cognitive impairments can make it more challenging to handle stressful situations effectively, as we're less able to think clearly or make sound decisions. A good night's sleep helps maintain normal cognitive function, enabling us to respond rather than react to stressful situations.
Recovery from Exercise
Sleep is key for sustaining physical energy and facilitating exercise recovery. Without it, your workouts suffer, and your exercise routines won't contribute as much to growth. Adequate sleep prevents injuries and exercise burnout because of the unique interplay of all the benefits of sleep listed thus far - proper immune function, restoration of tissues, improved autonomic tone (increased HRV), and much more.
In addition, short sleep durations can inhibit testosterone production, which contributes to muscle protein synthesis. Also, it is theorized that sleep contributes to your body's ability to store muscle glycogen, fuel for muscle contraction. Sleep deprivation may hamper this process and therefore inhibit exercise performance.
By prioritizing sleep, your body will have more adaptation fuel to commit to recovery from exercise, and you will be able to experience sustained improvement.
3 Sleep Variables to Focus On
Now that we’ve unpacked the role of sleep in improving adaptive capacity let’s shift our attention to how to improve sleep.
While sleep stages (e.g., REM, Deep, etc.) from wearables can provide some insights, they aren’t yet accurate enough to be relied upon. Instead, focus on these three sleep variables:
According to the National Sleep Foundation. Adults should aim for 7-9 hours of sleep each night. This is a hard and fast rule. And no, you probably aren’t there a person with the special genetic polymorphism that enables you to thrive on just 5 hours per night.
Consistently sleeping less than 7 and more than 9 hours each night is linked to a host of diseases. Sleep deprivation, sleeping less than 7 hours per night, is associated with an increased risk of obesity, diabetes, and type 2 diabetes, and all-cause mortality. The deregulation of key hormones such as growth hormone, cortisol, melatonin, and our hunger hormones, leptin, and ghrelin drives this.
Yes, when you go to bed matters. Sleeping from 10 pm to 6 am is NOT the same as sleeping from 1 am to 9 am.
In addition, a robust study involving over 800,000 people in the UK Biobank found that going to bed earlier and waking up early by 1 hour can reduce the risk of major depression by 23%. Shifting the midpoint of sleep 1 hour early decreased the risk of major depression by 23%, and another hour was about 40%.
The most likely reason that sleep timing dramatically impacts mood is that early risers get more exposure to natural sunlight - more on that in a moment.
Your sleep schedule is crucial for maintaining your circadian rhythm. Inconsistent sleep schedules can lead to social jet lag, a condition where your body's internal clock and social schedule are misaligned.
Social jet lag can increase your risk for weight gain, depression, cardiovascular disease, diabetes, and more. So find a sleep schedule you can follow during the week and on the weekends to prevent you from feeling like garbage on Monday.
Steps to Improve Your Sleep
Anchoring Your Circadian Clock with Early Morning and Frequent Daytime Sun Exposure
Every day, we ascend and descend a continuum of alertness and calm—our relationship with light drives this process. When you view sunlight early in the morning, it kicks off a cascade of hormonal and neurological events that directly impacts your energy, mood, and ability to adapt to stress.
Light from the sun is our primary zeitgeber or time-giver, anchoring our circadian clock. This light exposure signals to the SCN that it's time to wake up and be alert by increasing cortisol and body temperature. It also impacts melatonin secretion, a hormone that regulates sleep.
Aim for at least 10 minutes of natural light exposure in the morning, even if it's cloudy. Also, take frequent breaks throughout the day to get natural light exposure. Your brain and body will thank you.
Proper Sleep Hygiene
Maintaining a sleep-friendly bedroom environment can significantly improve your sleep quality. This includes keeping your bedroom cold, dark, and quiet like a cave.
Turn the temperature down in your bedroom about an hour before bedtime. This helps facilitate a smooth transition to sleep as your core temperature decreases during sleep. You can also consider taking a warm shower or bath to facilitate body cooling.
If light is an alerting signal during the day, it is also an alerting signal at night. Make your room as dark as possible. This includes turning your cell phone over and keeping the TV off. Even small amounts of ambient light can negatively impact blood sugar regulation, sleep quality, time spent in deep sleep, and more.
Finally, keep your bedroom quiet or use a white noise maker to maintain a constant tone. Abrupt changes in the ambient sound can pull you out of sleep and negatively affect your energy level upon waking.
Avoiding Caffeine Late in the Day
Caffeine is a stimulant that can interfere with your ability to fall asleep. The half-life of caffeine is approximately 5 hours in a healthy adult. Therefore, if you consume 200mg at 3 pm, 100 mg of caffeine is still active in your system at 8 pm.
For people that are caffeine sensitive, this can significantly disrupt a smooth transition from wakefulness to sleep. So be mindful of your caffeine consumption, and modify as necessary.
Avoiding Alcohol in the Evening
Finally, although alcohol consumption may help you fall asleep faster, it can disrupt your sleep later in the night and lead to frequent awakenings. Fractured sleep can significantly reduce your sleep duration and inhibit the restorative nature of sleep.
A recent study (2) found compared sleep quality among subjects who consumed various amounts of alcohol, and the results demonstrate that almost any alcohol consumption negatively impacts sleep quality:
- Low amounts of alcohol: Having less than 2 drinks per day for for men and less than 1 drink for women decreased sleep quality by 9.3%.
- Moderate amounts of alcohol: Having 2 drinks for men per day and 1 drink for women for women decreased sleep quality by 24%.
- High amounts of alcohol: Having more than 2 drinks per day for men and more than 1 drink for women decreased sleep quality by 39.2%.
Sleep is a powerful tool for enhancing adaptive capacity. It restores the body, detoxifies the brain, improves emotional regulation, enhances recovery from exercise, and balances hormones. By focusing on the three key sleep variables - duration, timing, and consistency - and implementing strategies to improve sleep, such as anchoring your circadian clock with early morning and frequent sun exposure, maintaining proper sleep hygiene, and avoiding caffeine late in the day and alcohol in the evening, you can significantly enhance your adaptive capacity.
Remember, sleep is not just about quantity but also about quality. Make sleep a priority, and your body and mind will thank you.
- Doghramji, Karl. "Melatonin and its receptors: a new class of sleep-promoting agents." Journal of Clinical Sleep Medicine 3.5 suppl (2007): S17-S23.
- Pietilä, Julia, et al. "Acute effect of alcohol intake on cardiovascular autonomic regulation during the first hours of sleep in a large real-world sample of Finnish employees: observational study." JMIR mental health 5.1 (2018): e9519.
Korem, Erik. “Accuracy of Consumer Wearable Technology” AIM7.com, March 8, 2023, https://aim7.com/post/accuracy-of-consumer-wearable-technology