A Wandering Mind is an Unhappy Mind
Research using a smartphone to randomly contact participants and quiz them about their thoughts and feelings throughout the day reveal that our minds wander quite a bit (47% of the time), and that on balance those wanderings are unpleasant. "The ability to think about what is not happening is a cognitive achievement that comes at an emotional cost." It would be interesting to do this research with a comparison sample of meditators.
Meditation Improves Mood in a Short Period of Time
Positive mood is associated with a left activated brain, activation referring to more amplitude at high frequencies (i.e. beta) as opposed to slower frequencies like alpha. Meditation has been shown to increase this asymmetry. For example, Richard Davidson had previously studied this with a standard 8 week program of MBSR, Mindfulness Based Stress Reduction, finding significant changes with an average of around 1 hour of meditation per day over 8 weeks. The newer study by Moyers shows changes with only around 10 minutes per day over 5 weeks. EDIT: Moyers discusses this research in a 3 part YouTube presentation.