Severe, recurring headaches often accompanied by nausea and sensitivity to light, migraines vary widely in frequency. Migraines affect women more often than men, and women are also more likely to have frequent migraines. The causes of frequent migraines vary widely from individual to individual.

Stress

Stress is a common migraine trigger. If a person experiences chronic stress, he may find himself subject to frequent migraine attacks.

Hormones

Hormonal changes, particularly in women, can bring on migraines. Many women find that they tend to get a migraine just before or during their period. Other women experience migraines as an unpleasant side effect of hormonal birth control.

Food and drink

Skipping a meal can trigger a migraine in some people. Others find that they're sensitive to certain foods. Common food triggers include caffeine, aspartame, red wine, cured meats and strong cheese. Dehydration can also bring on a migraine. Since exercise can also be a trigger, it's extra-important for migraine-prone athletes to monitor their fluid intake.

Sleep

Changes in waking and sleeping patterns can trigger migraines, too. Both not getting enough sleep and sleeping too much may have an effect.

Sensory overstimulation

Bright lights, unusual smells and loud noises can bring on a migraine attack. Fluorescent light is a common culprit; so is tobacco smoke.

Rebound

Sometimes overmedication is the problem. Pain medicines may mask the pain of migraines for a short while, but when they wear off, the headache returns. If too much is taken too frequently, the result will be a rebound headache -- the pain comes back again, only more severely. These headaches can be very hard to treat.

Source:

Mayo Clinic: Migraine -- Causes

MedicineNet.com: Migraine Headache

HealthCentral.com: Common Migraine Triggers