Have you ever wondered what the health impact of a stressful day was? Will you perform well during your long run or training session tomorrow morning? Is there anything you can do today that would improve your ability to have a better day tomorrow? HRV may be a piece of data that could help you answer these questions.

HRV is simply a measure of the variation in time between each heartbeat, also known as the RR interval. This variation is controlled by a primitive part of the nervous system called the autonomic nervous system (ANS). The ANS works regardless of our desire and regulates, among other things, our heart rate, blood pressure, breathing, and digestion. The ANS is subdivided into two large components, the sympathetic and the parasympathetic nervous system, also known as the fight-or-flight mechanism and the relaxation response.

The brain is constantly processing information in a region called the hypothalamus. The hypothalamus, through the ANS, sends signals to the rest of the body either to stimulate or to relax different functions. It responds not only to a poor night of sleep, or that sour interaction with your boss, but also to the exciting news that you got engaged, or to that delicious healthy meal you had for lunch. Our body handles all kinds of stimuli and life goes on. However, if we have persistent instigators such as stress, poor sleep, unhealthy diet, dysfunctional relationships, isolation or solitude, and lack of exercise, this balance may be disrupted, and your fight-or-flight response can shift into overdrive.

HRV is a noninvasive way to identify these ANS imbalances. If a person’s system is in more of a fight-or-flight mode, the variation between subsequent heartbeats is low. If one is in a more relaxed state, the variation between beats is high. In other words, the healthier the ANS the faster you are able to switch gears, showing more resilience and flexibility. Over the past few decades, research has shown a relationship between low HRV and worsening depression or anxiety. A low HRV is even associated with an increased risk of death and cardiovascular disease (Buccelletti et al., 2009; Tsuji et al., 1994).

People who have a high HRV may have greater cardiovascular fitness and be more resilient to stress. HRV may also provide personal feedback about your lifestyle and help motivate those who are considering taking steps toward a healthier life. It is fascinating to see how HRV changes as you incorporate more mindfulness, meditation, sleep, and especially physical activity into your life. For those who love data and numbers, this can be a convenient way to track how your nervous system is reacting not only to the environment, but also to your emotions, thoughts, and feelings.

The gold standard to measure HRV is to analyze a long strip of an electrocardiogram, the test that occurs frequently in medical offices where wires are attached to the chest. But over the past few years, several companies have created heart rate monitors that sync with apps that do something similar. The accuracy of these methods is still under scrutiny, but the technology is improving substantially. A word of caution is that there are no agencies regulating these devices, thus they may not be as accurate as claimed to be. With that said, the easiest and cheapest way to check HRV is to buy a chest strap heart monitor (e.g., Polar H10) and download a free app (e.g., Elite HRV) to analyze the data. The chest strap monitor tends to be more accurate than wrist or finger devices. Check your HRV in the mornings after you wake up, a few times a week, and track for changes as you incorporate healthier interventions.

Tracking HRV may be a great tool to motivate behavioral change for some. HRV measurements can help create more awareness of how you live and think, and how your behavior affects your nervous system and bodily functions. While it obviously can’t help you avoid stress, it could help you understand how to respond to stress in a healthier way. While there are questions about measurement accuracy and reliability, if you decide to use HRV as another piece of data, do not get too confident if you have a high HRV, or too scared if your HRV is low. Think of HRV as a preventive tool, a visual insight into the most primitive part of your brain.

Far from the metronome we might assume it to be, the healthiest heart beat follows a fractal pattern, with varying lengths of time separating each pulse (Tapanainen et al., 2002; Yaniv, Y., Lyashkov, A. E., Lakatta, E. G., 2013). A higher HRV suggests a relaxed, low-stress physiological milieu, while a lower HRV indicates a need for recovery, rest, and sleep. Therefore, in order to increase HRV, generally speaking, a more relaxed, low-stress environment is desirable. While there are a number of ways to reduce stress and increase relaxation, here are some examples that have been observed to increase HRV:

• Allocate time for rest
• Drink green tea (or take L-theanine) (Fiorino et al., 2012)
• Don't procrastinate (Dimitriev, Dimitriev, Karpenko, & Saperova, 2008; Tharion, E., Parthasarathy, S., & Neelakantan, N., 2009)

• Reduce commuting time and overtime (Kageyama et al., 1998)
• Work a job with a high reward to work ratio (Hintsanen et al., 2007; Loerbroks et al., 2010)
  
• Practice forgiveness (Patel, 2013)
  
• Practice yoga (Patil et al., 2013; Tyagi & Cohen, 2016)
• Practice meditation (Krygier et al., 2013; Nesvold et al., 2011)
• Listen to calm, relaxing music (Da Silva, 2014; Valenti et al., 2013)
• Breathe deeply and slowly (Török, T., Rudas, L., Kardos, A., Paprika, D., 1998)
• Try forest bathing (Lee et al., 2011)
• Consume omega-3 fatty acids (Sauder et al., 2013)

Physiologically, fasting:

• Stabilizes blood sugar
• Reduces insulin levels and improves insulin resistance
• Recovers and regenerates the gastrointestinal and immune system
• Increases ketone production
• Increases metabolic rate
• Removes damaged cells
• Reduces hunger
• Reduces excess body fat
• Reduces level of hormones thought to promote cancer
• Reduces the rate of aging
• Sheds excess body fat
• Protects brain function

The only other strategy that has so many research-baked benefits for longevity is long-term calorie restriction, which requires a significant long-term reduction in the amount of food you eat so that you are essentially living on the brink of starvation. Compliance with calorie-restricted diets is abysmal. Fortunately, there are many ways to fast, and there is likely a form of fasting out there that you will be able to tolerate and incorporate into your life. It's important for you to remember that fasting can provide nearly identical benefits without the pain, suffering, and compliance challenges of calorie restriction.

Instead of regulating how much food you eat, as with long-term calorie restriction, you only need to modify when you eat - and of course wisely choose the foods you do eat.Simply cycling between periods of eating and fasting on a daily, weekly, or monthly schedule has been shown to provide many of the same benefits as long-term calorie restriction. Choosing when to eat and when to fast in this way is known as "intermittent fasting."

Intermittent fasting is rapidly growing in popularity for the simple reason that it works. It does so whether you're trying to loss excess body fat or improve biofeedback for optimal health. As a general rule, intermittent fasting involves cutting calories, in whole or in part, either a couple of days a month or a week, every other day, or even daily.
 
There are many ways to fast, from consuming nothing but water for two to three days each month to eating a normal amount of calories every day but during a restricted window of time so you still get a good long stretch without food intake during each 24-hour period.

Here's a brief overview of a couple of different options:

2- to 3-Day Water Fast
It is generally not recommended to go without food for any period longer than about 18 hours. However, if you are overweight and have serious health challenges, a medically supervised water fast may be appropriate.

A water fast consists of consuming nothing but water and some minerals for a finite period of time. This type of fast can give you a shorter transition into fat burning because the body will rapidly burn through glycogen stores and begin using fat for energy.

This fast may be appropriate if you have just received a very serious diagnosis, such as brain cancer. Although if you are limited by any of the following conditions, consult with your health care practitioner before embarking of this type of fast:

• Already underweight
• Nutritionally compromised
• Taking diuretics or blood pressure medications
• Have low blood pressure
• Have diabetes, thyroid disease, chronically low sodium levels, or cardiovascular disease

5-Day Fast
This fast consists of spending five consecutive days of each month on a modified fast. You do not abstain from food entirely during these days. On the first day, you eat about 1,000 to 1,100 calories, followed by 725 calories on the remaining four days. As with all fasting options, the foods you do eat should be low in net carbohydrates and protein, and high in healthy fats.

Researchers have observed when people who have fasted five consecutive days once a month for three consecutive months saw improvements in biomarkers for cell regeneration. Risk factors for diabetes, cancer, cardiovascular disease., and aging also declined (Brandhorst et al., 2015).

Beware that is can be quite challenging to go for a full five days with very little food, especially if you've never fasted before, so you may want to work your way up slowly to this type of fast. Remember, "low and slow."

1-Day Fast
With this fast, you skip eating for one day of every week, consuming only water on that day. Your fast should be broken with a regular-sized meal, and you can maintain your regular exercise program without any special diet recommendations for workout days.

Fasting for 24 hours can be rough for some people, but eating a high-fat low-carb diet can make a 24-hour fast easier, as a higher fat diet will tend to normalize your hunger hormones and provide improved satiety for longer periods of time. You can also fast from dinner to dinner, skipping a full 24 hours of eating while still eating each day.

Alternate-Day Fasting
This program consists of eating on one day, and not eating on the next. During fasting days you restrict your eating to one meal of about 500 calories. On nonfasting days, you can eat normally.

When you include sleeping time, your fast can end up being as long as 32 to 26 hours. According to Krista Varady, Ph.D., author of The Every-Other-Day Diet, alternate-day fasting can help you lose up to two pounds of body fat per week.

Another benefit to alternate-day fasting is that your body tends to adapt to the regularity of the program. In clinical trials, about 90% of participants were able to stick to alternate-day fasting, whereas the other 10% dropped out within the first two weeks.

Peak Fasting
This form of fasting is by far the easiest to maintain once your body has shifted over from burning sugar to burning fat as its primary fuel, and it also appears to support steady circadian rhythms.

Peak Fasting is done every day rather than a few days per week or month. However, you can certainly cycle in off days to suit your schedule or social commitments - this flexibility is another major benefit of Peak Fasting. If circumstances allow, it is recommended implementing this type of fasting about five days a week. The process is quite simple.

The crux of Peak Fasting is to restrict your eating each day to a 6- to 11-hour window. As a result you will avoid eating for 13 to 18 hours every day. The simplest way to implement this type of fasting is to stop eating at least three hours before bed and then delay your first meal of the following day until at least 13 hours have passed since you last ate.

This may seem like an awfully long time to go without eating on a daily basis, but once you have transitioned to burning fat as your primary fuel, you won't experience those frequent hunger pains. Another benefit of Peak Fasting is that you will be able to go hours without a dip in energy because fat provides a continuous source of fuel. This is in contrast to glucose, which triggers glucose/insulin spikes, frequent hunger pains, and energy crashes as cues to consume more high-carb foods.

Regardless of the fasting program you choose, or even if you choose not to adapt any formal type of fasting, you should stop eating at least three hours before you go to bed. This simple change can help optimize your mitochondrial function and prevent cellular damage. Many factors influence why you will reap health benefits if you develop the habit of not eating within three hours of bedtime:

• When you are sleeping, your energy needs are at their lowest, and providing excess fuel at this time will result in the production of excessive amounts of damaging free radicals.
• Sleep is your body's time for detox and repair, and needing to digest a meal during sleep will impair these important processes.
• Nighttime is a common time for your body to use ketones for energy, since glycogen stores are typically depleted within 18 hours (13 hours if you are eating low quantities of carbohydrates), and eating too close to bedtime can replenish glycogen stores and prevent the body from burning fat for overnight fuel.
• Not eating for at least three hours before bed enables you to extend that period of time without eating food on a daily basis.

A compilation of scientific literature published in 2011 provides much of the experimental work supporting the advice from eating too close to bedtime. The message is clear: since the body uses the least amount of calories when sleeping, eating close to bedtime adds excess fuel which will generative excessive free radicals that will damage the tissues, accelerate aging, and contribute to chronic disease (Pamplona, 2011).

Although intermittent fasting, particularly Peak Fasting, is a powerful way to improve your physiological function all the way down to the mitochondrial level, it is not for everyone. Individuals taking medications, especially those with diabetes, need medical supervision; otherwise there is a risk of hypoglycemia.

If you have serious adrenal challenges or chronic renal disease, are living with chronic stress (adrenal fatigue), or have cortisol dysregulation, you would likely need to resolve these issues before implementing intermittent fasting. Also, if you have a disease called porphyria, you should not fast.

If you goal is to build large muscles or engage in competitive sports such as sprinting that require glucose for anaerobic fast twitch muscle fibers, intermittent fasting is not likely to be your best strategy.

Pregnant women and nursing mothers should not practice intermittent fasting, as the baby needs a wider range of nutrients during and after birth.

Children under 18 should also not fast for extended periods. Moreover, anyone of any age with concerns of malnutrition, or who is underweight (with a BMI less than 18.5), or who has an eating disorder such as anorexia nervosa should avoid fasting.

When implementing intermittent fasting, keep your eye on any signs of hypoglycemia, or low blood sugar, which include:

• Light-headedness
  
• Shakiness
  
• Confusion
  
• Fainting
  
• Excessive sweating
  
• Blurred vision
  
• Slurred speech
  
• Feelings of an atypical heartbeat
  
• Pins and needles sensation (neuropathy) in the fingertips
  

If you suspect that your blood sugar is low, make sure to eat something that will not impact your blood glucose levels, such as foods with a lower glycemic index quantity, including coconut oil in black coffee or tea.

The most challenging part of any intermittent fasting plan is getting through the initial transition, which can take anywhere from a week to two months. In some people this transition may take even longer, depending on how insulin resistant they are and other factors such as weight, blood pressure, and how consistent they are with their fasting regimen.

Roughly 10% of people report headaches as a side effect when they first begin fasting, but the biggest complaint is hunger. This is why it is so important to remain hydrated, especially while adding extra magnesium. It may be helpful to remember that one reason you're craving food is because your body has not yet made the switch from burning sugar to burning fat as its primary fuel. As long as you're running on sugar, frequent hunger pains will be the norm. Fat is far more satisfying, as it's a much slower-burning fuel.

Another factor that can cause disruption during the transition period is purely psychological. If you're used to grazing in the evenings, it may take some time to break the habit. One trick is to make it easier to go longer periods of time without eating is to drink more water. Oftentimes people mistake thirst for hunger.

It typically takes a few days to work up to 13 hours of fasting, but once you start activating your fat-burning system you will easily achieve this. The most effective way is to keep to your fat-burning plan by limiting your net carbs to under 40 grams per day and not exceeding more than 1 gram of protein per kilogram of lean body mass.