Hi, Jan. I can only tell you what I observe in my own daughter, since she's the one with Moyamoya (Daphne's two years old, and had surgeries last March). I know that she sleeps a lot more than other children her age, and whenever I go places with her, people are always saying "Awww, she looks so tired."
I also notice that she sleeps more on rainy and cloudy days (I'm more tired on those days, too!), and I notice that following her surgeries - slllowwly - she does seem to sleep less during the day, and more at night (hallelujah!). For a few months after her surgeries she was sleeping so much. I did wonder if it was a "growth" thing, maybe new blood vessells were growing into her brain

Outside, in really bright sunlight or cold weather, she has always just "shut down". Within a few minutes of being outside, she'll fall asleep but wake up as soon as we're inside again.
I have no idea what, if any of this, is related to her moyamoya. It does seem like I've read on this board that other Moyamoya children get more tired than "normal" kids, and that some days are better than others.
Jenny