While diazepam can help you sleep, it is generally not recommended for insomnia, especially right before exams. Diazepam is a long-acting medication, meaning it remains in your body for a long time. It is highly likely to cause next-day drowsiness, dizziness, and reduced alertness, which can impair your concentration and memory during your tests. For your exams, it is safer to avoid starting this medication. Please consult your doctor to discuss safer, short-acting alternatives or relaxation techniques to help you sleep. Good luck with your exams!