When all sleep, including naps, for a 24-hour period for 2-year-olds was compared with the nocturnal sleep of 5-year-olds (who do not nap), it was found that the minutes and percentages of stage 4 and the percentages of rapid eye movement (REM) sleep were very similar.
Previous investigators had reported a striking reduction in REM sleep and increase in stage 4 between ages 2 and 5 but they had not included nap time in their computations.
Kahn E, Fisher C, Edwards A, Davis DM. 24-Hour Sleep PatternsA Comparison Between 2- to 3-Year-Old and 4- to 6-Year-Old Children. Arch Gen Psychiatry. 1973;29(3):380-385. doi:10.1001/archpsyc.1973.04200030068010