Sleep debt
The hours you sleep less than you need, piling up on top of each other. Like small debts that grow over time.
A little more detail
The recommended sleep duration for adults is 7 hours or more per night on a regular basis (joint AASM and SRS statement). In the experiment by Van Dongen and colleagues, in healthy adults whose time in bed was restricted to 4 or 6 hours for 14 days, cognitive performance got progressively worse in a dose-dependent way; 6 hours of sleep or less a night created a deficit equivalent to 2 nights without any sleep. The participants' sleepiness ratings, however, rose only slightly; that is, people largely did not notice the deficit.
Source: Watson NF, Badr MS, Belenky G et al. Recommended amount of sleep for a healthy adult: a joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society. J Clin Sleep Med. 2015;11(6):591-592. doi:10.5664/jcsm.4758; Van Dongen HPA, Maislin G, Mullington JM, Dinges DF. The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation. Sleep. 2003;26(2):117-126. doi:10.1093/sleep/26.2.117