Cortisol is a steroid hormone which is essential for survival. It is responsible for the breakdown of fat and protein and stimulates the production of glucose in the liver. It is often referred to as the fight or flight hormone as it is key to stress response. Cortisol is produced by the adrenal gland, located just above the kidney, alongside other hormones such as adrenaline.
Cortisol is involved in several metabolic and immune processes in the human body. When the body is under stress it releases cortisol causing our levels to rise. 
Some studies have shown that people who are high cortisol reactors i.e. those who release high amounts of cortisol in response to stress, have a greater snack intake in relation to stress than low reactors. Therefore, stress and cortisol may play a role in weight gain and obesity. 
Under a normal sleep pattern Cortisol release peaks when we wake up in the morning and reduces throughout the day. This is controlled by a section of the brain called the hypothalamus which is like the control centre of the body. However, the pattern of cortisol release and declination can be changed by various activities throughout the day. For example, exercise can cause an increase in cortisol levels in the blood which also leads to the release of glucose and fatty acid stores for energy. Or it could rise in response to a mental stressor such as anxiety. 
Our body clock and the internal circadian clock can influence cortisol levels too. Sleep deprivation is a physiological stressor and can result in an increase in cortisol levels. If the circadian clock becomes misaligned – inappropriately timed sleep - for a prolonged time, then this can result in a decrease in cortisol. 
When the body is exposed to stressful situations communication between the nervous and endocrine (hormones) system occurs. As a result, the adrenal glands will release more cortisol into the blood alongside other stress hormones from other glands within the body. There are a number of events which the body classes as stressful including sleep deprivation, restriction of calories, too much exercise and other mental states. As a result of stress, the body’s normal circadian release of cortisol is disrupted and can lead to a number of illnesses. 
Cortisol is at its highest level when we wake up and at its lowest when we go to bed. Therefore, the timing of a cortisol test is very important.
When your cortisol levels are high, it is normal to have a change in mood i.e. feeling down, anxious or moody. You may also gain weight quickly which can affect your confidence and self-esteem. Other symptoms can include difficulty concentrating, reduced libido and high blood pressure.
Low cortisol levels, on the other hand, can make you feel weak, tired and can reduce your blood pressure, all of which can make you feel all out of energy. Low cortisol causes a low sodium.
An inability of the body to produce more cortisol in response to stress, as in Addison’s, can be critical therefore low morning levels should be investigated by your GP.
Lifestyle changes can help to control cortisol levels. Diet, exercise and stress-relieving exercises can all help to keep stress and cortisol levels down. Mindfulness-Based Stress Reduction Programmes can help to improve the way the body deals with stress. Exercises such as yoga, Pilates and meditation may be beneficial, too. If you can try to take yourself away from the situation awhich is causing you stress.
The foods you put into your body can impact many of its functions. For example, uncontrolled blood glucose levels and high levels of inflammation can cause high cortisol levels. Therefore, you should focus on foods which contain good fats such as fish, lean meat, avocados, eggs, nuts and olive oil. You should also increase your intake of dietary fibre too with foods such as wholegrains, fruit and vegetables.
Caffeine can also increase cortisol levels. Therefore, you should try to limit the amount of caffeine you drink throughout the day. 
Mid to high intensity exercise can increase cortisol levels because it places stress upon the body. Low intensity exercise doesn’t increase cortisol levels significantly. 
However, exercise is believed to be key to managing stress and reducing anxiety and depression.  You should aim to exercise for 30-60 minutes most days of the week to feel the full benefits of exercise.
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 Newman, E., O’Connor, D, B and Conner, M. (2007). Daily Hassles and Eating Behaviour: The Role of Cortisol Reactivity Status. Psychoendocrinology: 32, pp 125-132.
 Lovallo, W, R., Farag, N, H and Wilson, M, F. (2006). Cortisol Responses to Mental Stress, Exercise and Meals Following Caffeine Intake in Men and Women. Pharmacology, Biochemistry and Behaviour: 83(3), pp 441-447.
 Wright, K, P., Drake, A, L and Czeisler, C, A. (2015). Influence of Sleep Deprivation and Circadian Misalignment on Cortisol, Inflammatory Markers and Cytokine Balance. Brain, Behaviour and Immunity: 47, pp 24-34.
 Head, K, A and Kelly, G, S. (2009). Nutrients and Botanicals for Treatment of Stress: Adrenal Fatigue, Neurotransmitter Imbalance, Anxiety and Restless Sleep. Alternative Medicine Review: 14 (2).
 Hill, E, E., Zack, E., Battaglini, C., Viru, M., Viru, A and Hackney, A, C. (2008). Exercise and Circulating Cortisol Levels: The Intensity Threshold Effect. J Endocrinol Invest: 31(7), pp 587-91.
 Harvard Health Publishing Harvard Medical School. (2011). Exercising to Relax. Available at: https://www.health.harvard.edu/staying-healthy/exercising-to-relax