Stress and Hair Loss
Yes, stress can cause hair loss. Your entire system is regulated by hormones and stress can seriously alter the normal interaction of hormones. For a more indepth explanation, see this Mayo Clinic article.
Medical Conditions / Medications
Medical conditions and some medications can cause hair loss.
If you experience consistent or unusual hair loss (as opposed to breakage) see your doctor. And if you are taking medications, check for that side effect. Sometimes you can switch to a similarly acting drug that doesn’t cause you to lose hair.
You can’t fight genetics. Some people will suffer hair loss not matter what.
Taking Care of Breaking Hair
Since hair breakage is often a big factor, it’s good to discern if the hair you see in the sink or in your brush is “lost hair” or broken hair. “Lost” hairs will have a bulb attached at the end growth end. Broken hairs do not. If breakage is your problem. treat your hair gently. That means not tugging or using hard plastic or metal brushes, warm vs hot water for shampooing, turning down the heat on the hair dryer or simply air drying, using gentle shampoos and making sure you aren’t using hair coloring products that are too strong for your hair. (Some drug store hair colorings are only for dark hair, so pay attention to labeling.)