Colic, Read as a Neurological Phenomenon: What the Crying Curve Is Actually Telling You

Lead

Every evening, right on schedule, it starts. You try holding the baby. You try feeding. You change the diaper. Nothing works. An hour passes, then two. At some point you realize you are close to tears yourself.

This phenomenon — called yukigure naki (evening crying) in Japan, and colic in clinical literature worldwide — has an uncanny regularity about it. It typically begins around two weeks of age, peaks somewhere around six weeks, and disappears on its own by three or four months. In the meantime, those responsible for a colicky baby pass through what can feel like the worst hours of every day, without understanding why it is happening.

This article approaches colic as a neurological developmental phenomenon rather than a sign that something is wrong. The argument is not consolation — it is a framework. Understanding what the nervous system is doing during this period helps set realistic limits on what a caregiver can do, and helps establish a boundary around when to step away.

The "3-3-3 Rule" and Where It Came From

The classic clinical definition of colic traces back to a 1954 paper by pediatrician Morris Wessel and colleagues, published in Pediatrics [1]. Following 98 infants enrolled in the Yale Rooming-In Project longitudinally, Wessel defined "paroxysmal fussing" as crying or fussing that persists for more than three hours a day, more than three days a week, for more than three weeks [1]. This became the Wessel 3-3-3 criterion.

In that original cohort, roughly half of the infants (48 out of 98) were classified as "fussy," with most showing paroxysmal fussing beginning around the second week of life and resolving by approximately eight weeks [1]. The word colic has traditionally carried connotations of gastrointestinal pain, but Wessel himself made no causal claim. The criterion was a descriptive definition of an observed behavioral pattern — a distinction that continues to be misread.

A 2017 systematic review and meta-analysis by Wolke and colleagues in the Journal of Pediatrics, covering 28 studies and 8,690 infants, filled in a more current picture [2]. Using modified Wessel criteria, colic prevalence was 17–25% at one to two weeks of age, remained elevated through six weeks, dropped to 11% at eight to nine weeks, and fell sharply to 0.6% by ten to twelve weeks [2]. The review also found that the often-cited "peak at six weeks" is not well supported at the population-average level: mean fuss and cry duration was stable at 117–133 minutes per day through six weeks, then declined to 68 minutes by ten to twelve weeks [2].

In other words, the explanation that "colic always peaks at six weeks" oversimplifies the data. What the evidence does consistently support across multiple independent studies is that most cases resolve naturally by around three months [1,2].

The Nervous System Perspective

Why does intense, difficult-to-soothe crying concentrate in a specific developmental window? A definitive answer does not yet exist, but several neurological hypotheses have been proposed.

One is the central nervous system sensory hypersensitivity hypothesis. Because colic is confined to the early postnatal period and resolves spontaneously, it has long been discussed as "transient developmental dysmaturation" [3]. Recent neuroimaging research suggests that infants who develop colic show higher central nervous system reactivity to sensory stimulation from birth onward compared to non-colicky infants [3]. As the nervous system matures, excessive reactivity settles — and this timeline roughly tracks the behavioral arc of crying observed in longitudinal studies.

A second hypothesis involves autonomic nervous system regulation. Research using measures such as heart rate variability has produced mixed findings, and there is ongoing discussion about whether colic represents an expression of brainstem-level autonomic and vestibular dysregulation [3]. At the same time, at least one study found no significant difference in autonomic balance between colicky and non-colicky infants [4], which makes a single physiological marker explanation unlikely.

What matters is that all of these hypotheses converge in the same direction: colic as a developmental phenomenon rather than a disease state. The Period of PURPLE Crying program, developed by Barr and colleagues, repackaged this perspective for caregivers [5]. The acronym PURPLE stands for: Peak of crying, Unexpected, Resists soothing, Pain-like face, Long lasting, and Evening. The core message is that intense, inconsolable crying in early infancy is a normal stage of development — not a sign of harm, and not a sign of failure.

Accepting the Limits of What a Caregiver Can Do

The PURPLE program was not designed as general public health messaging. It was designed as an abuse-prevention intervention specifically targeting abusive head trauma. That clinical context matters.

In a randomized controlled trial published in the Canadian Medical Association Journal, Barr and colleagues found that mothers who received PURPLE educational materials within two weeks of delivery (n=1,279) were significantly more likely than control-group mothers to leave the room when they could not soothe the baby (0.067 episodes per day versus 0.039 per day), and also scored higher on knowledge about the dangers of shaking [6]. A parallel replication RCT conducted in Japan confirmed improvements in both knowledge and self-reported behavior [7].

The point worth emphasizing: walking away is a medically recommended response, one that has been studied in randomized trials and shown to have a measurable protective effect. It is not negligence. It is active safety behavior.

The limits of what caregivers can do are finite, and this is a medical reality rather than a personal failing. When the crying is not stopping, placing the infant safely in the crib and stepping away for a few minutes — or handing off to another caregiver — is the rational response to a neurological phenomenon that cannot currently be switched off.

The effect of colic on caregiver mental health also deserves acknowledgment. Elevated rates of postpartum depression, parenting stress, and reduced parenting self-efficacy have been observed in mothers of colicky infants [8]. "This is not your fault" is not simply a consolatory phrase. It is a claim that the developmental nature of colic and the intervention research behind the PURPLE program repeatedly support from the data side [5,6,8].

What This Looks Like Day to Day

Tomorrow's practical options come down to roughly three things.

1. Visualize the timeline: Keep a rough log of when the crying starts and how long it lasts. Looking at the pattern over several days in a record-keeping app like Memori makes it easier to see that crying concentrates in the evening, and to notice when last week's peak was slightly shorter than this week's. Logging is useful here less as a clinical tool than as a way of anchoring "when will this end?" into a time axis instead of an open-ended dread.

2. Decide in advance when to step away: Agree with your household on a rule — for instance, if thirty minutes of soothing has not worked, place the baby somewhere safe and take a few minutes out of the room. Decisions made calmly in advance are far easier to follow at two in the morning than decisions made in the middle of it.

3. Consult your pediatrician if crying meets the 3-3-3 criterion, is accompanied by vomiting or fever, or if weight gain is poor: Colic is a diagnosis of exclusion — it requires ruling out other medical causes first. If crying has persisted to the Wessel threshold, the cost of a consultation is reliably worthwhile [1,2].

Summary

Evening crying and colic are increasingly understood as a developmental phenomenon reflecting transient immaturity of the nervous system [3,5]. The 3-3-3 criterion is a descriptive definition, not a causal diagnosis [1]. Prevalence data show that the pattern resolves for most infants by around eight to nine weeks, with a sharp further decline through ten to twelve weeks [2].

Crying that cannot be stopped does not reflect a caregiver's competence. The act of stepping back when stopping is not possible has been tested in randomized trials as a shaken baby prevention strategy, and it works [6,7].

The two hours that feel endless this evening will, by most accounts in the data, be somewhat shorter next week.

References

1. Wessel MA, Cobb JC, Jackson EB, Harris GS Jr, Detwiler AC. Paroxysmal fussing in infancy, sometimes called colic. Pediatrics. 1954;14(5):421–435. PMID: 13214956.
2. Wolke D, Bilgin A, Samara M. Systematic Review and Meta-Analysis: Fussing and Crying Durations and Prevalence of Colic in Infants. J Pediatr. 2017;185:55–61.e4. doi:10.1016/j.jpeds.2017.02.020. PMID: 28385295.
3. Darque A. Shedding light on excessive crying in babies. Pediatric Research. 2020. (review of neurodevelopmental and central nervous system hypotheses of infantile colic)
4. Kirjavainen J, Jahnukainen T, Huhtala V, et al. The balance of the autonomic nervous system is normal in colicky infants. Acta Paediatr. 2001;90(3):250–254. PMID: 11332162.
5. Barr RG. The Period of PURPLE Crying: a new way to understand the variability and 'normality' of crying. Pediatr Child Health. National Center on Shaken Baby Syndrome program documentation.
6. Barr RG, Rivara FP, Barr M, Cummings P, Taylor J, Lengua LJ, Meredith-Benitz E. Effectiveness of educational materials designed to change knowledge and behaviors regarding crying and shaken-baby syndrome in mothers of newborns: a randomized, controlled trial. CMAJ. 2009;180(7):727–733. doi:10.1503/cmaj.081419. PMID: 19255028.
7. Fujiwara T, Yamada F, Okuyama M, et al. Effectiveness of educational materials designed to change knowledge and behavior about crying and shaken baby syndrome: a replication of a randomized controlled trial in Japan. Child Abuse Negl. 2012;36(9):613–620. PMID: 22954642.
8. Vik T, Grote V, Escribano J, et al. Infantile colic, prolonged crying and maternal postnatal depression. Acta Paediatr. 2009;98(8):1344–1348.