Crying and stress

Infants cry for many reasons: hunger, tiredness, discomfort, overstimulation or a need for closeness. During the first months of life crying increases gradually, often peaking around 6–8 weeks of age before declining. This pattern is sometimes called the "Period of PURPLE Crying" and is universal – it has been observed in every culture studied.¹ Around 10–20 % of all infants cry so much that they meet the criteria for colic, usually defined as more than three hours per day, at least three days per week.² Crying also tends to cluster in the late afternoon and evening, a phenomenon sometimes called the "witching hour."¹

Colic is not a disease but the upper end of a normal spectrum. Despite the name deriving from the Greek word for the large intestine, colic has no connection to gastrointestinal problems in the vast majority of cases.³ Infants with and without colic show the same type of crying and the same daily pattern, just in different amounts.³ The variation between infants is large: at the peak, a quarter of all infants cry more than 3.5 hours per day, while another quarter cry less than 1.7 hours.³ What drives the most stress in parents is not the total amount of crying but the episodes when the baby is inconsolable, roughly 5–10 % of all crying episodes, which can last 40 minutes or more.³ In over 95 % of cases no medical cause for the crying is found. It is instead explained by the normal maturation of behavioural states during the first months of life.³

A common worry is that you "spoil" the baby by comforting too often. Research does not support the idea that responsive comforting is harmful. Bilgin & Wolke (2020) analysed data from over 1,000 children and found that high responsiveness to crying did not lead to worse behaviour or insecure attachment at 18 months of age.⁴ Responsive comforting during the first months of life is, on the contrary, associated with reduced crying later in infancy.¹

However, research shows that the degree of parental involvement at sleep onset – such as rocking or nursing the child to sleep – is associated with more night wakings.⁵ This is not about comforting being harmful in itself, but rather that children who become accustomed to a high level of help to fall asleep may find it harder to fall back asleep independently between sleep cycles.

Cortisol is a hormone released in response to stress. The most frequently cited study in this area is Middlemiss et al. (2012), which measured cortisol levels in 25 infants during extinction-based sleep training (the child was left to cry without comforting).⁶ The study found elevated cortisol levels in the infants and that mothers' cortisol levels decreased faster than the infants'. However, the study has important limitations: it lacked a control group, had a small sample and has not been replicated in larger studies. Gradisar et al. (2016) conducted a larger, controlled study with an extinction-based method and found no elevated cortisol levels.⁷

Research clearly distinguishes between brief, situational crying and chronic, untreated stress. Chronic stress with elevated cortisol levels over time can negatively affect brain development.⁸ Brief crying in a child who otherwise has a secure and responsive caregiving relationship has not been shown to have the same effects. It is the overall pattern of care that matters, not individual episodes of crying.⁹