All of us need ways to manage the stress we face in our lives. Here is a helpful tool - the mindfulness bell. I like to use it in my middle school history classes. For the duration of the sound of the bell, try to focus your thoughts only on your breathing. Perhaps you could count your breaths or zero in on the sensation of air coming into and leaving from your body.
This may be a challenge at first. It certainly was (and still is) for me! Foreign thoughts try to force themselves into my consciousness. Yet, the more I practice, the better I get at focusing my attention on my breathing.
When you have mastered this task, try to maintain your focused attention for ever longer periods of time. You will probably find that this relaxes your brain and body, lowering your stress level and improving your ability to think in depth.