The healthiest time of day to workout is in the morning, between the hours of 6-10 AM. This is because the morning is when our cortisol (alertness/stress hormone) is highest naturally, so aligning physical exertion with this time allows you to take advantage of that cortisol spike. If you workout later in the day when your cortisol is lower, it forces your adrenal glands to pump out cortisol, which puts strain on your adrenals which can contribute to fatigue. If your workout is within 3-4 hours of before you sleep, it also messes up your natural sleep/wake cycle because your cortisol will be abnormally high when you try to sleep, and so sleep quality will be worsened. Second, the early morning hours are when you can get valuable morning light exposure, which improves sleep and increases natural melatonin levels in the body at night. It has also been shown to decrease depression symptoms and seasonal affective disorder. Even walking outside in the morning can have profound improvements in sleep that night, and this type of light exposure is especially great for those whose adrenal glands have been overworked and worn out because it helps them get back into the natural cycle of spiking cortisol in the morning and slowly decreasing throughout the day to allow for optimal sleep and energy the next day. In this picture this athlete is wearing Nike shoes, Lulu Lemon Leggings, an Apple Watch and a Lulu Lemon shirt. You can visit these sites by going to nike.com or Lululemon.com
