The crossword solution for the term 'Reason the Physicist Stayed in Bed?' is INERTIA, which is a seven-letter word. INERTIA is a scientific concept that states an object at rest will remain at rest unless acted upon by an external force. This concept was first proposed by Sir Isaac Newton in his first law of motion. Therefore, the reason the physicist stayed in bed could be due to INERTIA, as he may have been too lazy to get up and move.