Some people have circadian rhythm sleep disorders which cause them to sleep much too early (Advanced sleep-phase syndrome, ASPS), much too late (Delayed sleep-phase syndrome, DSPS) or in several bouts throughout the day and night (Irregular sleep-wake disorder). The worst circadian rhythm sleep disorder in people who can see is Non-24-hour sleep-wake disorder (Non-24); their "daily" rhythm lasts 25 hours or more so they sleep and wake up one or more hours later each day. Blind people who see no light at all can also have Non-24, but it is less serious and can usually be treated.