Study design. One week before the experiment, participants were instructed to maintain a regular sleep–wake rhythm. They were given an actigraph to objectively monitor their sleep–wake rhythm. During this week, participants completed sleep and consumption diaries to ensure adherence to the study protocol. On the experiment day, participants came to the laboratory in groups of three to play the PGG. Afterward, they were fitted with a high-density portable EEG system and sent home where the sleep EEG was recorded during the following night.