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Cortisol.  The stress hormone.  When under stress, the adrenal glands (yes, the same ones that make adrenaline) release this hormone.  It's principle function is to increase blood glucose levels through various processes, thereby producing energy.  This hormone sometimes receives a bad rap for increasing fat stores, but that's only one function of this hormone.

For the most part, this hormone is involved with immune suppressing and metabolic processes.  These processes are pivotal in providing energy during stressful situations.  When I refer to stress, this can be anything that illicts your anxiety.  So you can have a slight bump in cortisol when you are preparing for a test, for a baby to arrive, or even if your child is late coming home.  These are perceived threats to yourself and thus your body responds, and one way is to release cortisol.

Obviously the perceived threats aren't physical in this case.  From an evolutionary perspective (we're not getting into that discussion), when we as humans had to roam and gather or hunt our foods and we faced real life threats from things like other humans, large animals, or storms, this hormone acted in our favor.  Now, with modern society, abundance of this isn't exactly a good thing.  Mobilizing extra glucose, especially when you don't need it in your body has a tendency to allow for more fat to be deposited.  Obviously this isn't desired.

Some good points to consider are being in a fasted state.  If you fast, cortisol is released which in turn allows for your body to make more glucose, of which it wasn't getting from your diet at that point.  It also helps with glycogen production (basically glucose in the muscles).  These are desired effects during a fasted state due to the lack of incoming nutrition.  Some of the reason why you can reduce weight or fat mass while fasting.

Now you see how stress can play a role in this.  Some of it can be beneficial and ultimate produces the desired effect (see previous paragraph).  On the other hand, abundance of cortisol can weaken the immune system, increase fat storage, and thus make you generally unhappy and miserable.

Finding ways to cope with your stress can help you reduce your cortisol levels.  Left unchecked, and you may find yourself in a position you don't want to be in.