This brief Scientific American article discusses how the brain responds to meditation. A few positive benefits are increased capacity for learning and memory, as well as the possibility for decreased anxiety and stress response. Interestingly, the brain structure itself is what changes, showing our amazing capacity to evolve and grow when we choose to do so. http://www.scientificamerican.com/podcast/episode.cfm?id=mediation-correlated-with-structura-11-01-22
While this article focuses on the meditator's benefits and the scientific findings of the a meditation practice, I'd like to add that any meditation practice is best held with the intention "how can I serve the whole, and choose to act in a way that will serve my highest motivations and qualities?", rather than meditating to improve one's feeling state or mental acuity alone.
In my experience, meditation is not always a relaxing experience, but it is always a growth experience. It often reveals the level of daily chatter that isn't contributing towards any forward motion. When I speak to patients about meditation, they often say they aren't relaxed enough to meditate, or they are too distracted by all the thoughts that come up.
The point of meditation is not to instantaneously achieve a state of deep relaxation, although it is often a side effect! The point is is to let go, and choose to have no relationship with your history or future, and the emotions, drama, and upset entangled therein.
How? In meditation, when a thought, feeling, or physical sensation comes up, you don't even need to acknowledge it. You simply choose not to have a relationship and focus on something very simple: nothing. It may not be comfortable, but again, not the point. In a culture where "nothing" is avoided at all costs are we are always doing and striving for "something," it may come as a pleasant relief or a shock to the system. Either way you will find yourself becoming more attuned to the place inside you where there is always a palpable sense of the still. It is practice for real life. So that when your emotions run high or you find yourself lost in a mental loophole, you can choose to act from a place of no-relationship, or stillness, rather than from a conditioned reaction.
And, you just might find that symptoms, such as headaches, tired eyes, and muscle tension - the signs of stress and anxiety - will fade, the positive correlations between meditation and the body detailed in the article above are your body's way of telling you what's good for you.