Step 1: Sit tall in your chair.
Step 2: Extend your right arm toward the ceiling as you inhale. Imagine you're reaching for an apple on a high tree branch.
Step 3: Maintain the reach and slowly exhale with the intention of sending your hip and thigh down into your chair.
Step 4: Repeat 5 times on each side.
Sitting compresses the diaphragm, and if you don't ever adjust your posture while you sit, you actually take in less oxygen with each breath you take. That's why you can feel tired by midday.