The Mayo Clinic said mental illness can be more than just feelings of sadness or distress. They may cause you to feel “significant tiredness, low energy or problems sleeping” according to the source.
Of course, insomnia will likely only lead to feeling more tired. Depression and chronic fatigue syndrome are often mistaken for each other, but can also exist at the same time, points out other sources. It’s important to let a doctor make that call to properly treat it.