Recently on our (Old) forums, a user who was new to meditation complained that they were feeling lightheaded and dizzy during meditation and had to stop. They then asked whether or not they were doing something wrong.
First, we need to understand what causes lightheadedness. According to Wikipedia, light-headedness can simply be caused by a shortage of oxygen or blood to the brain, low blood sugar, or hyperventilation.
So why would meditation cause this? When we meditate, our pulse and breathing rate slow down. Sometimes, however, we make purposely slow down our breath before our body is ready for this low breathing rate. This could, in theory, cause lightheadedness while meditating. So what can be done about this?
Let your body breath naturally. Do not try to control the breath. Let your body inhale and exhale naturally. No force is necessary. Your breath will slow down on its own; there is no need to slow it down manually. If you do slow it down manually, it could cause this lightheadedness.
Interestingly, the forum user altered their breathing to a more natural rhythm, and had a successful meditation session with no lightheadedness.
If it is not the breathing, however, that is causing lightheadedness, try drinking some water before meditating. Also, I personally get lightheaded when I haven’t eaten anything for awhile. If you have not eaten anything in a while, try eating something, and in theory, your lightheadedness should go away (If it doesn’t perhaps you should seek professional help).
So in conclusion, breath naturally while meditating; don’t try to shorten it on purpose. Doing so may make you lightheaded.
If you remain lightheaded during meditation, or at any other time during day, perhaps seek professional help.