Due to a lack of oxygen.
Before it rains, there is often a noticeable drop in atmospheric pressure. In response, the circulatory system begins to lower blood pressure and reduce heart rate, which in turn leads to a decrease in oxygen flow to the tissues.
Although this does not affect the functioning of most organs, the nerve tissue of the brain is extremely sensitive to the level of oxygen in the blood. Even a slight decrease causes a reduction in brain activity, which a person perceives as drowsiness.
