The out of sorts feeling you get when you’ve been up three times in the middle of the night with a crying or feeding baby and the next day you have a headache, upset stomach and general disorientation. Baby lag is like jet lag, only worse because there is a cumulative effect of sleep deprivation. A 2012 study commissioned by Johnson Baby Products of 1000 new parents showed by about 50% got only four hours of sleep per night.
— As cited in Wired Magazine 2/13 and the MacMillan Dictionary