The Quick Answer
You've probably read that when a baby stays awake too long, their body floods with cortisol and adrenaline, giving them a "second wind" that makes sleep impossible. It's one of the most repeated ideas in baby sleep. When we went to the research, we couldn't find evidence for it. Overtired babies really are fussier and harder to settle, and that's worth preventing. But it isn't a stress-hormone emergency, and one short nap or late bedtime won't physically harm your baby.
If you've ever lain awake worrying that a missed nap was "flooding" your baby with stress hormones, this post is for you.
I'm Sarah Mitchell, founder of Helping Babies Sleep and author of The Helping Babies Sleep Method. I've worked with more than 1,000 families across Palo Alto, San Jose and the Bay Area, and I've watched this one idea cause more anxiety than almost any other piece of sleep advice. So our team went back to the studies. Here's what we found, what we didn't, and what it means for your baby.
The Claim You've Probably Read
Read almost any baby sleep blog, parenting site or even a diaper brand's sleep tips and you'll find some version of this:
- "When a baby is overtired, cortisol and adrenaline flood their little body."
- "That's why overtired babies get a second wind."
- "Every minute past the wake window raises their stress hormones."
It sounds scientific, and it feels true when you're holding a baby who is screaming at 9:30 p.m. But when you look for the research behind it, the citations are usually missing. When there is one, it often points to a study that didn't measure cortisol at all.
I'll be honest: our own blog does speak to overtiredness but never the flood of cortisol. Part of being a good sleep consultant and thought leader is going back and checking what we all "know." This is one of those times, and we've since updated those posts with what we've found. Let's dive in.
What Cortisol Actually Does
Cortisol isn't a villain. It's a normal hormone that rises and falls every single day naturally in our circadian rhythms. This is true in every baby, toddler and adult.
It has a daily rhythm. A large 2025 analysis of infant studies found that the morning cortisol peak gradually emerges from birth and the daily pattern is well established by about 6 to 9 months. Source This ties into the phenomenon of the circadian rhythm being immature at birth and fully maturing around 6 months of age.
- It naturally rises when we wake up. This is called the cortisol awakening response. In a study of 150 toddlers and young children, 142 showed this normal morning rise. Source
- It helps us get up and going. Higher in the morning, lower at night: that's healthy.
So the question isn't whether babies have cortisol. They do. The question is whether staying awake too long sends it surging. That's where the evidence falls apart.
What the Research Actually Shows
1. In adults, the effect is real but limited
A well-known 1997 study kept healthy young men awake all night or cut their sleep short. Their cortisol was about 37 to 45 percent higher the following evening, and the normal evening drop started later. Source That's a real finding, but notice what it is: a significant loss of a whole night's sleep, in adults, with an effect the next day. It isn't a same-day surge from a 20-minute late nap.
2. When researchers skipped toddlers' naps, they didn't report a surge
This is the closest test of the claim we found. Researchers measured cortisol in 2.5 to 3 year olds on days they napped and on a day their nap was skipped. Napping produced a small cortisol rise after waking, just like the morning rise. Skipping the nap did not produce a reported cortisol spike. Source
A 2022 review that pooled studies of children from birth to 5 years came to a similar conclusion: cortisol fell across the day in children who napped and in those who didn't, and naps played only a minor role in children's daily cortisol. Source And in a study of children in childcare, cortisol often rose over the day, but nap length and quality weren't related to it. Source
3. In babies and toddlers, we only have associations
There is some link between sleep and cortisol in young children, and it's important to be honest about it:
- Toddlers with more fragmented sleep had higher cortisol on waking. But how long they slept wasn't linked to cortisol at all, and the authors note their design can't show which causes which. Source
- Meaning it wasn't keeping them awake too long that cause the spike but rather all the interrupted night sleep.
- Meaning it wasn't keeping them awake too long that cause the spike but rather all the interrupted night sleep.
- In one-year-olds, babies who fell asleep later had higher cortisol measured over several months. But day to day, a short night didn't predict higher cortisol the next day. Instead, higher cortisol predicted shorter, later sleep. In other words, a baby's cortisol pattern may shape their sleep, not the other way around. Source
- The long view: babies who usually fell asleep later had higher cortisol in their hair. That’s a link, but it doesn’t tell us which came first.
- The day-to-day view: this is where it gets interesting. A short night didn’t lead to higher cortisol the next day. It worked the other way: on days a baby’s cortisol was higher, they slept less that night, and higher cortisol at bedtime meant they fell asleep later.
- What it means: it’s a chicken-and-egg question, and in this study the cortisol looked like the chicken. A baby’s natural cortisol rhythm may influence how easily they fall asleep, rather than a late bedtime causing a cortisol surge.
- In 8-year-olds, chronically poor sleep was linked to differences in cortisol and stress-system activity. Source That's older children, ongoing poor sleep, and an association, not a bedtime surge.
- Meaning chronically poor sleep was linked to differences in cortisol, not having a slightly later bedtime one day.
These studies tell us that sleep and the stress system are connected over time. They don't show that keeping a baby awake past their wake window triggers a cortisol spike that day or night.
4. Nobody has measured the adrenaline
We could not find a single study measuring adrenaline in overtired babies or toddlers. The adrenaline half of the claim appears to have no research behind it at all.
So Why Does My Overtired Baby Seem So Wired?
You're not imagining it. Overtired babies and toddlers really do fall apart. The research simply points to different explanations.
- Tired little (and big) people have a harder time regulating emotions. In a study where 2.5 to 3 year olds missed their nap, they showed about a third fewer positive reactions to a puzzle they could solve and about a third more negative reactions to one they couldn't. Source That's "fussy" and "falling apart," measured.
- Timing against the body clock. In toddlers, the sleep hormone melatonin starts rising roughly 45 minutes before their usual bedtime, and about one in five toddlers in one study were being put to bed before it had started. Source Toddlers put down before their body clock was ready took 40 to 60 minutes to fall asleep and resisted bedtime more. Source
- This could be explained by the idea of "sleep pressure" - the homeostatic system, putting kids down before their awake windows have been reached may fight sleep.
- This is also why a baby's morning wake-up time matters so much. A baby who woke at 8:00 a.m. often isn't ready for a 6:30 p.m. bedtime, no matter how cranky they seem. I explain how to match bedtime to your baby's day in Wake Windows by Age.
- This could be explained by the idea of "sleep pressure" - the homeostatic system, putting kids down before their awake windows have been reached may fight sleep.
- Fussy babies are harder to soothe. A cranky baby needs more help to calm down, which means more rocking, more bouncing and more feeding to sleep. That's often the real cycle parents get stuck in.
Why This Myth Matters
If this were just a harmless figure of speech, I wouldn't spend a whole post on it. But I see it keep good parents stuck every week.
- It turns every short nap into an emergency. Parents afraid of "causing stress hormones" rescue every nap, rock longer and never give their baby room to practice falling asleep.
- It makes sleep teaching feel dangerous and prevents parents from moving forward. A few days of imperfect naps while your baby learns a new skill will not harm them.
- It adds guilt to an already exhausting season. You are not damaging your baby when a nap goes sideways.
We covered a shorter version of this in our myth check in Why Babies Need Motion to Sleep, because the fear of cortisol is one of the biggest reasons parents feel they can't step away from rocking and bouncing and continue to "make sleep happen."
What About Cortisol and Sleep Training?
The cortisol fear gets even louder when parents think about sleep teaching: won't the crying flood my baby with stress hormones? It's a fair question, and it's one researchers have actually tested.
In 2016, sleep researcher Dr. Michael Gradisar and his colleagues in Australia published a randomized controlled trial in the journal Pediatrics. They followed 43 babies, 6 to 16 months old, who had sleep problems. Families were randomly placed into one of three groups:
- Graduated extinction: putting baby to bed awake and checking in at gradually longer intervals.
- Bedtime fading: moving bedtime later so it matched when the baby was actually ready to fall asleep.
- Control: families who received general sleep information only.
The researchers measured the babies' stress with saliva cortisol samples taken in the morning and afternoon. Cortisol did not go up in the sleep training groups. If anything, it showed small-to-moderate declines compared with the control group. The babies in both sleep training groups also fell asleep faster and woke less often, and at 12 months there were no differences between the groups in emotional or behavioral problems or in how securely the babies were attached to their parents. Source
A few things to keep in mind. This is a small study, so like everything in this post, it isn't the final word. The babies were 6 months and older with established sleep problems. But it's one of the few studies that actually measured cortisol in babies while they learned a new way to fall asleep, and it doesn't support the idea that sleep teaching floods babies with stress hormones.
Notice the bedtime fading group, too. Simply matching bedtime to when babies were biologically ready to sleep improved their sleep. That's the same body-clock idea behind matching bedtime to your baby's morning wake-up time in Wake Windows by Age.
This doesn't mean you need to leave your baby to cry. The Helping Babies Sleep Method is a responsive, step-by-step approach that gets the timing and feeding right first, so there's far less protest when it's time to teach the skill. You can see how it compares in Ferber vs. CIO vs. The Helping Babies Sleep Method.
What We're Not Saying
Being careful with the evidence goes both ways, so here's what this post doesn't claim:
- We're not saying overtiredness doesn't matter. It makes babies more upset, makes sleep harder and is worth preventing.
- We're not saying sleep and cortisol are unrelated. Long-term poor sleep is linked to differences in the stress system. What's missing is evidence for a same-day surge from a missed wake window.
- We're not saying the question is closed. The studies in young children are small. "Not shown" is the honest answer, not "disproven." If better research comes along, we'll update this post.
If your baby is persistently inconsolable, snores, has pauses in breathing or seems unwell, talk to your pediatrician. That's never a timing problem to solve on your own.
What to Do Instead: Timing for the Right Reasons
In The Helping Babies Sleep Method, timing is the second of five pillars. We care about it because good timing builds healthy sleep pressure, keeps your baby calmer and makes falling asleep easier. Not because we're racing a stress hormone.
- Use age-appropriate wake windows as your starting point, then fine-tune by 10 to 15 minutes. Here's the wake window chart by age.
- Anchor the day to morning wake-up time. Bedtime follows from when your baby started their day.
- Move bedtime earlier after a nap drop, use awake windows to guide bedtime vs a fixed time that suits a family's schedule.
- Don't panic about one rough day. Get back on track with the next wake window or an earlier bedtime.
- Build the skill. A baby who can fall asleep on their own copes far better with an off day. The Two Phases of Sleep Training explains how we sequence that work.
Not sure whether you're dealing with overtiredness, a regression or something else? Start with Regression or Schedule Problem? How to Tell the Difference, or read the 5 signs your baby is overtired.
Sleep Help in Palo Alto and the Bay Area
If you're tired of second-guessing every nap, we can help. We support families across Palo Alto, Menlo Park, Mountain View, Los Altos, Sunnyvale, Cupertino and San Jose, and coach families virtually anywhere.
- Not sure where to start? Take the free Sleep Quiz →
- Want the wake window chart? Download the free Sleep Chart →
- Baby 4 to 24 months? Explore baby sleep coaching →
- Toddler 2 to 4 years? Explore toddler sleep coaching →
- Ready to talk about coaching? Book your intro call →
Frequently Asked Questions
Do overtired babies really get a cortisol surge?
There's no good evidence for it. No study we could find has kept a baby awake past their wake window and measured a cortisol spike. When researchers skipped toddlers' naps and measured cortisol, they didn't report a surge, and a review of nap studies found naps play only a minor role in children's daily cortisol.
Do overtired babies get an adrenaline rush?
We couldn't find a single study that measured adrenaline in overtired babies or toddlers. The adrenaline part of the story appears to have no research behind it at all.
Why does my overtired baby get a second wind?
Overtired babies and toddlers are genuinely more emotional and less able to cope, which can look wired. Timing also plays a part: research in toddlers shows that a bedtime that doesn't match their body clock can mean 40 to 60 minutes of fighting sleep. That's a timing and regulation problem, not a hormone emergency.
Is it harmful if my baby takes a short nap or has a late bedtime?
An occasional short nap or late bedtime won't harm your baby. Overtiredness is worth preventing because it makes babies fussier and sleep harder, but one rough day doesn't flood your baby with stress hormones. Get back on track with the next wake window or an earlier bedtime.
Is there any link between poor sleep and cortisol in children?
Yes, some. Studies have found that children with more fragmented or poorer sleep tend to have different cortisol patterns. But these studies show associations, not cause and effect, and one infant study suggests cortisol may shape sleep rather than the other way around. That's very different from a same-night surge after a missed nap.
Should I still follow wake windows if overtiredness isn't a cortisol emergency?
Yes. Wake windows matter for the right reasons: healthy sleep pressure, a calmer baby and an easier put-down. Timing is the second pillar of The Helping Babies Sleep Method, and it's often the first thing we fix.
Does sleep training raise a baby's cortisol?
Not according to the research we have. In a 2016 randomized controlled trial led by Dr. Michael Gradisar, published in Pediatrics, babies aged 6 to 16 months who were sleep trained did not show higher cortisol than babies in a control group. Their cortisol showed small-to-moderate declines instead, they slept better, and at 12 months there were no differences in emotional or behavioral problems or attachment. It's a small study, but it's one of the few that measured cortisol in babies during sleep teaching.
Keep Reading
- Wake Windows by Age: A Simple Guide for Parents
- Why Babies Need Motion to Sleep
- The Helping Babies Sleep Method: The Complete Overview
- Toddler Bedtime Battles: How to Create Calm, Consistent Evenings
- The Science of Baby Sleep
About the Author
Sarah Mitchell, Kin, DC (retired) is the founder of Helping Babies Sleep, a Bay Area sleep consulting agency based in Palo Alto, and the author of the Amazon bestseller The Helping Babies Sleep Method: The Art and Science of Teaching Your Baby to Sleep. A retired chiropractor, Certified Lactation Counselor, RIE Foundations graduate and member of the Society of Behavioral Sleep Medicine, she has supported more than 1,000 families over 13 years.
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