Is there a blood test for testing excessive daytime somnolence?

No: There are many causes of excessive daytime sleepiness. An overnight sleep study is one test but is not used in all cases. I would see a sleep specialist who can better determine the right studies to help determine what is going on.

Excessive somnolence: If you have daytime sleepiness, there is a screening tool called the epworth sleepiness scale. If you get a score greater than 10, you have have a sleep problem and should see a sleep specialist to review. Need a sleep study. There are some blood tests that are commonly done when someone is tired all the time such as a complete blood count or thyroid function test, which help determine the cause.