Workout and sleep, what's best for me?

There are many benefits to regular exercise, including a lower risk of diseases like cancer and diabetes, improved physical function, and a higher quality of life.

Exercise can also have more direct benefits for certain groups. For example, elderly people who exercise regularly are at a lower risk of being injured during a fall and pregnant women who routinely exercise are less likely to gain excessive amounts of weight or experience postpartum depression.

Furthermore, exercising can improve sleep for most people. Engaging in moderate-to-vigorous physical activity can increase sleep quality for adults and reduce sleep latency whilst alleviating daytime sleepiness and reducing the need for sleep medication.

There have been many sleep studies exploring the relationship between exercise and sleep with strong evidence that exercising more has undeniable benefits for sleep. However, not many have explored the impact of workout timing and sleep from a fundamental basis.

Is it harmful to exercise before bed?

The traditional view is that intensive exercise during the three-hour period leading up to sleep can negatively impact sleep because it can increase your heart rate, body temperature and adrenaline levels. However, other studies have noted that exercising before bed may not produce any negative effects.

One study found that most people who exercise at 8pm, or later fall asleep quickly, experience adequate deep sleep and wake up feeling refreshed. Respondents exercising between 4 and 8pm reported similar results.

However, the results are mixed. One researcher found that a higher core temperature was associated with lower sleep efficiency and more time awake after sleep onset. So while exercising before bedtime may not be inherently harmful, vigorous exercise in the hour before bed can impact sleep efficiency and total sleep time by raising your core temperature.

When we exercise, our body experiences significant changes

Firstly, blood is diverted from your liver and digestive system to your skeletal muscles. Hormones tell the body to convert fat into glucose, reduce the pain you feel and improve your mood. Muscles generate lactic acid as a byproduct of intensive exercise and, as this builds, the pH of the blood around the muscles drops.

The brain makes neurotransmitters, like serotonin, dopamine and GABA. This is part of the reason why the brain consumes more energy during exercise. Our adrenaline levels rise, stimulating the heart to beat faster. The muscles of the ribcage assist the diaphragm to pull in more oxygen and breathing gets faster and deeper. Waste heat is carried away as you sweat.

Afterwards, your body will experience an "overreaching" when it resets after its elevated state

Immediately after exercise your body will try to return to its resting state as quickly as possible, and the fitter you are the better your body is at doing this. Typically during this process your body experiences an "overreaching" effect where it overcompensates for the state of elevation it was in before.

Why do we sleep?

• Energy conservation: we sleep to conserve energy. It allows us to reduce our caloric needs - essentially spending part of our time functioning at a lower metabolism or resting when its inconvenient and less efficient to hunt for food.
• Cellular restoration: we sleep to restore our body, to allow cells to repair and regrow. Functions include muscle repair, protein synthesis, tissue growth and hormone release
• Brain function: sleep allows neurons to reorganise. Whilst you sleep your brain clears out waste from the central nervous system and also converts short-term memory into long-term memory as well as erasing unnecessary information.

What happens to our body when we sleep?

Your body cycles through four stages of sleep, these cycles occur multiple times throughout the night for different lengths of time varying from 70 to 120 minutes each.

• Stage 1: Non-REM sleep: When you first fall asleep, as your body enters light sleep, your brain waves, heart rate, and eye movements slow down. This phase generally lasts about 7 minutes.
• Stage 2: Non-REM sleep: Light sleep, just before deep sleep. Body temperature falls more, eye movements stop and heart rate and muscles relax more.
• Stage 3: Non-REM sleep: Deep sleep. Eyes and muscles don't move, and your brain waves slow down even further. This sleep is restorative and your body replenishes energy and repairs cells, tissues, and muscles.
• Stage 4: REM sleep: About 90 minutes after you fall asleep, you'll enter REM sleep. During this stage your eyes move quickly from side to side, brain waves and eye movements increase and your heart rate and breathing also speed up. Dreaming often occurs during REM sleep, and your brain processes information during this stage

How does a smartwatch determine what sleep stage you are in?

Most smartwatches measure sleep quantity and quality using accelerometers & gyroscopes built into the device. By measuring your wrist movement whilst you're asleep they can predict your sleep time and quality. However, this is limited in that it doesn't give accurate information on your sleep stages. Fortunately, smartwatches also take information on your heart rate and respiration allowing them to accurately predict your REM sleep as well as deep and light sleep stages.

Additional data such as the length of time of your movements are indicative of your sleep behaviour (such as rolling over) and help confirm that you're asleep. Whilst sleeping, your device tracks the beat-to-beat changes in your heart rate or HRV which is a useful predictor of which sleep stage you're in.

Therefore, whilst exercising can influence your sleep there is no ‘one size fits all' solution for when you should exercise. For some people, their personal preference might be to exercise in the mornings whilst others may prefer to exercise in the evening.

With this in mind, see what works for you and how it impacts your sleep. For example, if you find you're struggling to sleep after exercising late at night then it might make sense to try working out in the morning instead and see how that influences your sleep.

Of course, there are plenty of other factors that influence your sleep (as discussed here) so it's important to think about what other habits may be influencing your sleep alongside your workout schedule.

How can we use exercise to influence our sleep?

Recall that our body produces serotonin when we workout and also experiences an "overreaching" effect when it resets back to normal levels. Given this, you can optimise your workout timing so that the overreaching effect is coincidental with when you'd like to sleep.

For example, if you're fit and exercise regularly then you'll be able to time an evening workout closer to when you'd like to sleep given that your body will reset at a fast rate.

One study actually found that a rapid decline in core body temperature favours sleep onset and also entry into deeper stages of sleep.

However, not all exercise is equal!

In general, it's best to focus on light to moderate-intensity exercise such as yoga, stretching, walking, and light to moderate weightlifting. This is because doing vigorous exercise like running, HIIT training, heavy lifting will leave your body in an elevated state for a longer period of time and make it more difficult for your body to reach its resting state to enable you to fall asleep.

The future of sleep and wearable technology

One of the biggest challenges with understanding the relationship between workouts and sleep is that there is little consistency across individuals. Because of this, it means we can't just apply a golden rule to everybody and say ‘exercise at least 4 hours before sleeping and you will sleep better'. For some people this may work, but for others this might not. For this reason, wearables become extremely useful - with extensive information on the history of workout data and sleep data whilst providing other information on other habits including eating we can paint a picture of what impacts us most. We will be able to answer questions like "When is the best time for me to workout?", "What sort of workouts help me sleep best?", and many more...