When I am stressed, I feel very alert due to my increased heart rate. This also makes me have insomnia, especially when I have not yet resolved the cause of my stress. I also experience loss of appetite.
These manifestations happen when we are stressed because stress triggers the fight or flight response. This response involves the secretion of adrenaline and cortisol by the adrenal glands, thus causing increased heart rate that increases alertness. The increased alertness makes it harder to sleep, as melatonin and cortisol have an inverse relationship. The stress hormones also slow down digestion and decrease hunger and appetite so more resources of the body can be used to combat the stressor.