Keeping a sense of balance requires the brain to process a variety of information received from the eyes, the nervous system, and the inner ears.
If the brain is unable to process these signals, such as when the messages are contradictory, or if the sensory systems are improperly functioning, an individual may experience lightheadedness or dizziness.
Other causes are: altitude sickness, low blood sugar, hyperventilation, postural orthostatic tachycardia syndrome (increase in heart rate upon sitting up or standing), panic attacks, and anemia.
If any of these serious disorders are present, the individual will usually have additional symptoms such as chest pain, a feeling of a racing heart, loss of speech or a change in vision.
When people experience unusual or extreme heat for a long time, sweating excessively can cause a lack of some electrolytes, which in turn can cause lightheadedness.