Sleep debt accumulates across consecutive nights of insufficient sleep, and research — including a well-known 2003 study by Van Dongen and colleagues restricting participants to 4-6 hours of sleep for two weeks — found that cognitive performance kept declining throughout the two weeks without leveling off, reaching levels comparable to a full night of total sleep deprivation, even though participants' own subjective sense of sleepiness plateaued and stopped tracking their actual, still-worsening impairment. That gap between how sleepy you feel and how impaired you actually are is part of why chronic mild sleep restriction is so easy to underestimate in daily life.
Social jetlag — the mismatch between a person's sleep schedule on workdays versus free days, catching up with several extra hours of sleep on weekends — provides some recovery but doesn't fully reverse accumulated deficits, and the resulting inconsistent schedule creates its own disruption to circadian rhythm, similar in miniature to jet lag from crossing time zones. The more effective approach isn't cramming recovery sleep occasionally; it's maintaining a consistent, adequate sleep schedule that doesn't accumulate debt in the first place.
Van Dongen's team restricted participants to either 4, 6, or 8 hours of time in bed per night for two full weeks straight, tracking cognitive performance daily. The 4- and 6-hour groups showed performance decline that never leveled off — by the end of two weeks, their impairment was comparable to a full night of total sleep deprivation. The striking, separate finding was that participants' own subjective ratings of how sleepy they felt stopped getting worse partway through the study, plateauing even as their actual measured performance kept declining for the full two weeks — a growing gap between how impaired people felt and how impaired they actually were.