Four-Month Sleep Regression Timing
Babies typically enter the four-month sleep regression between 3 to 4 months of age, coinciding with major developmental changes in sleep architecture and motor skills that occur during this critical period. 1
Developmental Context at 4 Months
The four-month period represents a critical developmental window when several physiological changes converge:
Sleep architecture matures significantly around 2-4.5 months post-term, when infants begin to exhibit clearly recognizable NREM sleep stages (N1, N2, and N3) with sleep spindles typically appearing by 2-3 months and slow wave sleep developing by 4-4.5 months 2
Motor development accelerates, with infants beginning to roll over generally between 4-6 months of age, which can disrupt established sleep patterns 1, 3
SIDS risk peaks between 1-4 months and then begins to decline after 4 months, reflecting underlying changes in neurological maturation and arousal mechanisms 1
Why Sleep Disruption Occurs at This Age
The convergence of multiple developmental factors explains the timing:
Neurological maturation of sleep-wake cycles occurs as the brain transitions from neonatal sleep patterns to more adult-like sleep architecture, with the dominant posterior rhythm increasing to 3.5-4.5 Hz by 3-4 months 2
Increased arousal capacity develops as protective mechanisms mature, making infants more likely to wake between sleep cycles 1
Physical capabilities expand as infants gain upper body strength and begin attempting to roll, which can wake them during sleep transitions 1, 3
Clinical Reassurance
Parents can be reassured that sleep disruptions around 4 months are developmentally normal and that the incidence of serious sleep-related risks like SIDS begins to decline after this age 1. The prevalence of parent-reported sleep problems is actually highest at 4-11 months (40.3%) compared to other age groups, confirming this is a common developmental phase 4.