"Research has confirmed what the yogis of ancient times already knew: Profound physiological and psychological changes take place when we meditate, causing an actual shift in the brain and in the involuntary processes of the body.
An instrument called an electroencephalograph (EEG) records mental activity. During waking activity, when the mind constantly moves from one thought to another, the EEG registers jerky and rapid lines categorized as beta waves. When the mind calms down through meditation, the EEG shows waves that are smoother and slower, and categorizes them as alpha waves.
As meditation deepens, brain activity decreases further. The EEG then registers an even smoother, slower pattern of activity we call theta waves. Studies on meditators have shown decreased perspiration and a slower rate of respiration accompanied by a decrease of metabolic wastes in the bloodstream. Lower blood pressure and an enhanced immune system are further benefits noted by research studies.
The health benefits meditation produces naturally reflect the mental and physical effects of this process. At the very least, meditation teaches you how to manage stress; reducing stress, in turn, enhances your overall physical health and emotional well-being. On a deeper level, it can add to the quality of your life by teaching you to be fully alert, aware, and alive. In short, it is a celebration of yourself. You are not meditating to get anything, but rather to look at and let go of anything you do not need."
Source: Mara Carrico, Yoga Journal, "A Beginner's Guide to Meditation", http://www.yogajournal.com/article/practice-section/let-s-meditate/