Today, millions of people worldwide play popular multiplayer online battle arena games (MOBAs) such as League of Legends. MOBAs are designed to require real-time teamwork to win games. A side effect of this is toxic behavior (an umbrella term for various negative in-game behaviors). Following Transactional Stress Theory, in this study, we consider toxicity as a coping mechanism to better deal with in-game stress that leads to anger and frustration, potentially leading to subsequent sleep problems. Therefore, we asked League of Legends players (N=212) about their experiences within the last 30 days in a retrospective survey study. Our results indicated that perceived stress was, indeed, a positive predictor of sleeping problems, and toxic behavior partially mediated this relationship. These results indicate that some of the toxicity is caused by stress in play and that it may increase the likelihood of the occurrence of subsequent sleeping problems. Therefore, further efforts are needed to help players to develop harmonious means and techniques for coping with experienced in-game stress.