NIGHTLY RECHARGE™

Nightly Recharge™ is an overnight recovery measurement that shows how well your body has coped with overall stress you have experienced lately. Overall stress can come from different sources including work, family, relationships, environment, lifestyle, training etc. Your body does not differentiate between the sources of stress. You may have had a demanding day at work, a high-intensity training session or a late-night party with friends. It is likely that after such a hectic day, your body is not able to recover well during the following night. Also, stress coming from things you perceive as positive, such as a late-night party, may add up overall stress to your body and compromise your nightly recovery. Your Nightly Recharge status is based on two components: how you slept (sleep charge) and how well your autonomic nervous system (ANS) calmed down during the early hours of your sleep (ANS charge). Both components are formed by comparing your last night to your usual levels from the past 28 days. Your watch automatically measures both sleep charge and ANS charge during the night.

You can see your Nightly Recharge status on your watch and in the CASIO WATCHS app. Based on what we've measured from you, you get personalized daily tips in the CASIO WATCHS app on exercise, and tips on sleep and regulating your energy levels on those particularly rough days. Nightly Recharge helps you make optimal choices in your everyday life to maintain overall well-being and reach your training goals.

How to benefit from Nightly Recharge?

Recovery plays a major role in performance development, optimal daytime functioning, well-being and health. It is not uncommon in our modern society that also people who train regularly, not aiming for the podium, feel fatigued or stressed out. Combining training with work, family and other commitments is sometimes demanding and there may not always be enough time for recovery.

Nightly Recharge helps you:

• see how your body is able to recover from training and stress
• by giving tips on exercise, sleep and regulating your energy levels
• learn how you can improve your night-time recovery

What does Nightly Recharge measure?

Nightly Recharge status is calculated by combining scores for both ANS charge and sleep charge. Both components are formed by comparing your last night to your usual levels from the past 28 days.

Nightly Recharge status has the following scale: very poor – poor – compromised – OK – good – very good.

ANS charge gives you information on how well your autonomic nervous system (ANS) calmed down during the night. The scale is from -10 to +10. Around zero is your usual level. Your ANS controls two sets of actions your body can take: whether it's time to fight or flight (sympathetic nervous system), or if it's time to rest and digest (parasympathetic nervous system). The higher the ANS charge, the higher the dominance of the rest-and-digest part of ANS compared to your usual level. ANS charge compares the recovery of your ANS to your usual level from the past 28 days.

ANS charge is based on the analysis of your heart’s beat-to-beat interval data, which is measured optically from your wrist. The three parameters forming the ANS charge heart rate, heart rate variability and breathing rate are measured during roughly the first four hours of your sleep. The first hours of sleep are more sensitive to reflect recovery than measurements averaged from a whole night's sleep. The first hours of sleep are also important for physical recovery as most of your deep sleep occurs then.

First your heart’s beat-to-beat interval data is converted into heart rate, heart rate variability (RMSSD) and breathing rate. Then these three parameters are combined into one single value to form your ANS charge value. Heart rate has the biggest influence when forming the score and breathing rate the smallest. The effects of stress on your body can be seen in the functioning of your ANS. After a demanding day, it takes more time than usual to switch from activation into relaxation mode, which is reflected in the ANS charge. To get a higher ANS charge your heart rate would need to be lower, heart rate variability higher and breathing rate lower than usual.

For example mental or physical stress, exercising late at night, illness, or alcohol can keep your heart rate up (and heart rate variability down) during the early hours of your sleep.

Heart rate is mainly regulated by the ANS. Your average heart rate during roughly the first four hours of sleep is measured. A normal value for adults can range between 40 and 100 bpm. It is common for your heart rate values to vary between nights. It's best to compare your last night's value to your usual level.

Heart rate variability (HRV) refers to variation in the time between successive heart beats. A higher HRV reflects the higher activation of the part of ANS responsible for resting and digesting. In general, high heart rate variability is linked to general good health, high cardiovascular fitness and resilience to stress. It can vary greatly from person to person, ranging from 20 to 150. It's best to compare your last night's value to your usual level.

Breathing rate shows your average breathing rate during roughly the first four hours of sleep. It is calculated from your beat-to-beat interval data. Your beat-to-beat intervals shorten when you breathe in and lengthen when you breathe out. During sleep, breathing rate slows down and varies mainly along with sleep stages. Typical values for a healthy adult at rest range from 12 to 20 breaths per minute. Your breathing rate doesn't normally vary a lot. Higher values than usual may indicate a fever or impending illness.

How sleep charge is measured?

Your Sleep charge looks at how you slept last night compared to your usual level. Your sleep score is a number that summarizes how much and how well you've slept. This score is compared to your usual scores from the past 28 days. The scale of the sleep score is 1 - 100. A typical sleep score is around 70 to 85.

Sleep charge = Sleep score compared to your usual level. Scale: much below usual – below usual – usual – above usual – much above usual. The green color means your charge was usual or better than usual.

A healthy adult normally goes through 4-5 sleep cycles over the course of a night. Normally sleep cycles proceed from light sleep into deep sleep, and then back to REM sleep. Sleep regeneration looks at the quality of your sleep in terms of the proportions of restorative sleep stages: REM sleep and deep sleep. REM sleep is important for learning and memory. Deep sleep restores your body. A healthy sleep structure contains a good amount of both REM sleep and deep sleep. Your usual level shows how this component was usually scored during the past 28 days. The components of the regeneration theme are evaluated against the indicators of good sleep quality based on the current sleep research literature.

• Sleep time: Sleep time is your sleep time minus interruptions.
• REM sleep %: REM stands for rapid eye movement. In REM sleep, your muscles are relaxed but your brain is active. REM sleep restores your mind, and enhances memory and learning. This is the stage when dreaming mostly occurs. Most REM sleep occurs during the second half of the night. The amount of REM sleep is on average about 21 % of sleep time for adults. 25 % gives the maximum score from this component of sleep score. Higher or lower percentages decreases your score.
• Deep sleep %: Deep sleep is the stage of sleep when it's hardest to be woken up. This sleep stage restores your body, supports your immune system and affects muscle growth. It also affects certain aspects of memory and learning. Most deep sleep occurs during the first half of the night. On average, deep sleep counts for about 15 % of sleep time for adults. Roughly 17 % gives you the maximum score for this sleep score component. A higher percentage does not improve your score.
• Light sleep %: Light sleep serves as a transition stage between wakefulness and the deeper stages of sleep. You can be easily awoken from light sleep since your responsiveness to the environmental stimuli remains quite high. Light sleep also promotes mental and physical recovery, although REM and deep sleep are the most important sleep stages in that regard.
• Long interruptions (min) tells the time you spent awake during the interruptions that were longer than 90 seconds. Interruptions in sleep are perfectly normal. During a normal night's sleep there are numerous short and long interruptions when you awaken from your sleep. Whether you remember these interruptions or not depends on their duration. The shorter ones we don’t usually remember. The longer ones, for instance when one might get up for a sip of water, we can remember. There are about 15 minutes of long interruptions in sleep during an average night. A night without any long interruptions gives the maximum

Scientific background

Nighty Recharge is based on up-to-date scientific knowledge on stress and recovery. It utilizes a generally accepted means to assess the function of the autonomic nervous system (ANS) and nightly sleep. Measuring the heart’s beat-to-beat intervals provides a valid tool to indirectly measure the ANS function as well as sleep in real-life settings.

ANS charge combines heart rate, heart rate variability and breathing rate smartly, in a way that reflects changes in the sympathetic and parasympathetic control of the heart. Heart rate is regulated by the both branches of the ANS. Activation of the sympathetic branch speeds heart rate up while activation of the parasympathetic branch slows it down. Variation in the heart’s beat-to-beat intervals (i.e. heart rate variability) is measured with a parameter called RMSSD (Root Mean Square of Successive Differences in beat-to-beat intervals). It especially reflects the parasympathetic control of the heart. The higher the parasympathetic activation, the higher the RMSSD value. Breathing rate can also be captured from the heart’s beat-to-beat interval data. It is typically quite stable from night to night but when it does deviate from its usual level, it adds meaningful information to the ANS charge.

Sleep charge is built on the SLEEP PLUS STAGES FEATURE. Polar’s automatic sleep measurement with the Sleep Plus Stages algorithm has been validated against polysomnography, which is the gold standard method to assess sleep. Sleep score is an easy to understand measure with a solid scientific basis. There is no single parameter that would indicate how well you slept, and therefore sleep score combines several components of sleep. These components have been formulated based on scientific sleep duration and quality recommendations.

Source: Polar Research
Polar science