Working out is one of the most effective stress relievers. Researchers recently found that after spending 30 minutes on a treadmill, their subjects scored 25 percent lower on tests that measure anxiety and showed favorable changes in brain activity. If you only have time to do one thing for yourself, make it sticking to your workout routines. If you can't hit the gym or trails, even a brisk 30-minute walk at lunch or getting up several times a day to stretch and walk around will help relieve stress.
I have read something different about theta waves and learning languages. A University of Washington study tested students resting brainwave activity before learning French. They found that students with a higher amount beta/gamma and a lower amount of delta/theta activity were better at acquiring a second language. When you are dominant in theta, that is the lowest and most deeply relaxed awakened state you can be in. I think it would be much harder to really concentrate, fully understand and learn new information while in a theta state, so I would personally consider using theta while studying.
The activity of neurons generate electric currents; and the synchronous action of neural ensembles in the cerebral cortex, comprising large numbers of neurons, produce macroscopic oscillations. These phenomena can be monitored and graphically documented by an electroencephalogram (EEG). The electroencephalographic representations of those oscillations are typically denoted by the term 'brainwaves' in common parlance.