Month 4: The Sleep Regression
The Quick Brief
Welcome to month four, where your champion sleeper suddenly forgets how to sleep. Don't panic—this isn't a step backward. Your baby's brain is leveling up, and their sleep architecture is maturing from simple newborn patterns to complex, adult-like sleep cycles. It's frustrating, but it's actually a sign of healthy development.
What's Happening with Baby
Around four months, your baby's brain undergoes a significant neurological shift. They're transitioning from the simple two-stage sleep of newborns (REM and non-REM) to the more complex four-stage sleep cycles that adults use. This biological change means your baby will now cycle between light and deep sleep every 60-90 minutes at night and 35-45 minutes during naps—and they may fully wake between each cycle.
Beyond sleep, this month brings exciting developmental milestones. According to the CDC, by four months most babies will hold their head steady without support, hold a toy when you put it in their hand, use their arms to swing at toys, bring hands to mouth, and push up onto elbows when on their tummy. Socially, your baby will smile on their own to get your attention, chuckle when you try to make them laugh, and make cooing sounds like "oooo" and "aahh." They're also becoming more aware of their environment, which contributes to the sleep challenges—they'd rather engage with the world than sleep through it.
What's Happening with Mom
By month four, many physical recovery aspects are complete, but this is when return-to-work decisions often become pressing. If your partner is heading back to work, the timing couldn't feel worse with sleep disruption hitting its peak. The sleep regression affects everyone—expect increased fatigue and potentially some tension as you both run on empty.
This is also a critical time for mental health monitoring. Postpartum depression and anxiety can emerge or intensify with sleep deprivation. Watch for signs that go beyond normal exhaustion: persistent sadness, loss of interest in the baby, excessive worry, or feelings of hopelessness. If your partner seems to be struggling more than typical new-parent tired, encourage her to talk with her doctor. The Edinburgh Postnatal Depression Scale is commonly used at pediatric checkups—dads can benefit from understanding it too.