It is linked to receiving radiation therapy to the head. The symptoms of somnolence syndrome usually happen 3 to 12 weeks after radiation therapy ends. It can last a few days or several weeks.

Does radiation cause sleepiness?

Most people getting radiation therapy feel fatigued. It does not mean that your cancer is getting worse. It does not mean that the treatment is not working. In fact, it is normal to feel very tired during this time.

How long does radiation induced fatigue last?

Radiation can give you fatigue that gets worse over time (called cumulative fatigue). It usually lasts 3 to 4 weeks after your treatment stops, but it can continue for up to 3 months.