Doctor Who Series 10: 12 Easter Eggs & References You Might've Missed In "Knock Knock'
9. "Sleep Is For Tortoises"
Bill questions whether the Doctor needs sleep in this episode, to which he replies: "Sleep is for tortoises." This is a direct quote from the Fourth Doctor, as said in 1977's The Talons of Weng-Chiang.
The Doctor also elaborates that he only sleeps after a regeneration or a particularly big lunch. The Doctor has indeed slept for extended periods after gaining a new body. Specifically, when he regenerated into his Third, Fifth, Seventh, Eighth, Tenth and Twelfth selves.
The "big lunch" incident the Doctor is referring to could well be 1986's The Two Doctors, in which the Second Doctor (who has been temporarily turned into a food-obsessed Androgum) dozes off after a hefty dinner.