I can agree to it because I experienced it before.
I have 2 reasons before:
1. It's either I can't sleep because I am excited about how to spend my winnings

2. Or I can't sleep because my winnings are not enough and I am still willing to play more, bet more, and expect to grow those winnings.
These are the exact things that could possibly cause sleepless nights for a gambler after winning, you'd either get really excited to gamble more or start overthinking about what to do with the amount of money won from the bet..one thing I've learnt is emotional control, whether I'm winning or losing I always try to keep myself calm because if you are not careful it's easy for you to mistakes at such point