The cause of a migraine remains unknown although there are some theories about this. One theory is that it is caused by vasodilation (blood vessel expansion) in the brain, one theory is that it is vasoconstriction (blood vessel narrowing) and another theory relates to a change in brain chemistry affecting blood vessels, nerves etc.
Migraines often occur in response to a trigger. These triggers are unique to each migraine sufferer and a key part of managing your migraines is to learn and manage your own triggers. There are many potential triggers for migraine, and to help you determine your own, a headache diary can be helpful in identifying what might be a trigger for you.
For example, many people find emotional stress and poor or disturbed sleep to be a trigger, others find the world around them can trigger a migraine such as harsh chemical smells and bright lights or flickering screens. Many find there are dietary triggers - such as missed meals, dehydration, alcohol (particularly red wine), caffeine as well as foods containing tyramine (such as cheeses). Women may find that normal hormonal changes (such as during the course of their normal period cycle) can trigger a migraine.