Eating Walnuts with your breakfast will provide you with energy for the day and give you the daily health benefits of walnuts.
Eating the Walnuts at night is also good for the health benefits and I would recommend eating the Walnuts at night before going to bed and in the morning with your breakfast to get the most nutrition and health benefits from the walnuts.
Eating Walnuts before bed can also help you get to sleep if you have trouble sleeping so that's another benefit of eating walnuts before bed.