Circadian rhythms aren't based solely on the sun. They are also based on social and physical activity. Things they don't get when sitting indoors for the entirety of their daylight hours.

Lack of sleep is primarily because they are not physically tired come bed time. They aren't physically tired becauae they haven't engaged in any physical activity all day. They've been obligated to sit indoors during daylight hours, and they don't have daylight for outdoor evening activities.

Instead, they get to sit indoors, under artificial lighting and watching bright screens, both of which screw up their sleep schedules.