As long you have been meditating, for 80 years or 2 weeks, all of us are encountering the extremes of drowsiness and agitation during the practice. Drowsy or agitated mind are the 2 most frequent obstacles in meditation.
From time to time, it is impossible to escape from them.
Mindfulness is paying attention moment- to- moment to ourselves and our environment w/o analytic and decisional purpose.
During the past 25 years or so neurosciences have been very productive in trying to understand how mindfulness works. But, like for any scientific research, some studies regarding mindfulness and mindfulness-based meditation are very good, others are lacking true scientific data and many questions remained unsolved.
Here is an update meta-analysis from Berkeley Univ.