Does Poor Sleep Flatten Your Stress Response? 295 Adults Tested

It can. In a new study of 295 young adults, people who reported worse insomnia symptoms showed a smaller cortisol rise, not a bigger one, when they were put through a stressful task in the lab. That surprises most people, because insomnia is usually described as being "wired". The story is more interesting than tired equals stressed, and it changes how you might think about recovery after bad nights.

What the new study measured

Iván Vargas and colleagues at the University of Notre Dame's Sleep and Health Research Lab announced the study in October 2026; it appears in Psychoneuroendocrinology. They brought 295 young adults into the lab, recorded their insomnia symptoms, then exposed them to a psychosocial stressor while collecting saliva samples to track cortisol, the body's main stress hormone.

The headline finding: greater insomnia symptom severity was linked to a significantly more blunted cortisol stress response. Gender and hormonal contraceptive use were also associated with flatter cortisol slopes, but the insomnia link held after the researchers controlled for both.

Two limits matter. The design was cross-sectional, so it shows an association at one point in time, not that insomnia caused the flatter response. And the sample was young adults, so it may not describe a 45-year-old founder or a parent of toddlers.

Why "blunted" is not the same as "calm"

It is tempting to read a smaller stress response as a good thing. Researchers see it differently. A healthy stress system rises when a challenge appears and settles when it passes. In a 2013 review in the International Journal of Psychophysiology, Phillips, Ginty and Hughes argued that unusually low or blunted cardiovascular and cortisol reactivity to acute stress, not only exaggerated reactivity, is associated with negative health outcomes.

So a flat line can mean the system is less responsive, not more relaxed. The Notre Dame authors point out that insomnia is often framed as hyperarousal, yet the evidence linking insomnia to cortisol has been "surprisingly mixed". Their result adds weight to the idea that, under acute stress, the system may react less rather than more.

What other sleep labs found

This is not the first time sleep loss has been linked to a changed stress response. The direction, though, has not always matched.

| Study | Who and what | Cortisol finding | |---|---|---| | Vargas et al., 2026 | 295 young adults; insomnia symptoms; lab psychosocial stressor | More severe insomnia, more blunted response | | Hansen et al., 2021 | 22 adults (11 normal sleepers, 11 with sleep-onset insomnia); 38 hours awake or control; Maastricht Acute Stress Test | Blunted peak after sleep deprivation, and in the insomnia group even when rested | | Vargas & Lopez-Duran, 2017 | 40 healthy adults aged 18 to 29; 29 hours awake or control; Trier Social Stress Test | Higher cortisol before the stressor, then a blunted response | | Minkel et al., 2014 | 26 healthy adults aged 22 to 49; one night without sleep or normal sleep; Trier Social Stress Test | Higher cortisol before the stressor and an amplified response |

The 2017 Notre Dame team offered one explanation for the mixed picture: if cortisol is already elevated after a sleepless night, there may be a ceiling on how much more a lab stressor can add. Minkel's group, by contrast, saw both higher resting cortisol and a bigger spike. Differences in timing, age, sample size and how sleep was lost could explain part of the gap. The honest summary is that sleep loss seems to disturb the normal rise and fall of the stress response, but not always in the same direction.

Why this matters beyond the lab

Insomnia is common everywhere. In a widely cited 2002 review in Sleep Medicine Reviews, Maurice Ohayon estimated that about one third of the general population reports at least one insomnia symptom, 9% to 15% have symptoms with daytime consequences, and about 6% meet criteria for an insomnia diagnosis.

For people who lead teams, a stress system that does not respond normally is relevant to focus and mental sharpness in the moments that count: the pitch, the negotiation, the hard conversation. The military-focused Hansen study made the same point about high-stakes, rapid-response work, suggesting blunted reactivity may reflect reduced resiliency.

A separate 2026 review in Translational Psychiatry by Maimon, Lerner and Admon looked at REM sleep and stress. It proposed that what matters most is not a single snapshot but the trajectory: an adaptive response may involve moderate changes in REM sleep after stress that resolve over time. Recovery, in other words, is about getting back to baseline.

What to do with this

None of these studies tested a fix, so treat the following as sensible steps rather than proven cures.

1. Take persistent insomnia seriously. If poor sleep has lasted weeks and is affecting your days, speak with a doctor. The American College of Physicians recommends cognitive behavioural therapy for insomnia (CBT-I) as the first treatment for chronic insomnia disorder in adults.
2. Protect recovery nights after high-stress days. Keep wake time consistent and give yourself a real wind-down window rather than working until lights out.
3. Remove easy sleep disruptors. A hot, humid bedroom is one of them. Whether you live in Singapore (we looked at why many adults there sleep under 7 hours on humid nights) or in a city facing hotter summers, a cooler room makes the rest easier.
4. Use comfort tools as comfort, not treatment. Some people find a weighted blanket helps them settle, and the evidence in adults with insomnia is mixed, as we covered in this review of weighted blanket insomnia trials. If heat keeps you awake, a cooling weighted blanket is one option; you can see how one is built on our specifications page.

For the related question of what evening cortisol does to the night ahead, see our look at pre-sleep cortisol and sleep.

The takeaway

Bad sleep does not simply make you more stressed. In several lab studies, it changed how the body's stress system responded to a challenge, and in the newest and largest one, worse insomnia went with a flatter cortisol response. The research is still mixed and mostly short-term, but the practical message is consistent: treat ongoing insomnia as something worth fixing, and give your body real recovery nights after hard days.

Sources

1. Vargas I, Dickens H, Vance A, et al. Insomnia symptom severity predicts blunted cortisol reactivity to a psychosocial stressor. Psychoneuroendocrinology. 2026;194:108033. https://doi.org/10.1016/j.psyneuen.2026.108033
2. Sleep and Health Research Lab, University of Notre Dame. New Publication Alert! (6 October 2026). https://sleeplab.nd.edu/news/new-publication-alert-7/
3. Phillips AC, Ginty AT, Hughes BM. The other side of the coin: blunted cardiovascular and cortisol reactivity are associated with negative health outcomes. International Journal of Psychophysiology. 2013. https://doi.org/10.1016/j.ijpsycho.2013.02.002
4. Hansen DA, Satterfield BC, Layton ME, Van Dongen HPA. Sleep deprivation and sleep-onset insomnia are associated with blunted physiological reactivity to stressors. Military Medicine. 2021. https://pmc.ncbi.nlm.nih.gov/articles/PMC10895409/
5. Vargas I, Lopez-Duran N. Investigating the effect of acute sleep deprivation on hypothalamic-pituitary-adrenal-axis response to a psychosocial stressor. Psychoneuroendocrinology. 2017;79:1-8. https://doi.org/10.1016/j.psyneuen.2017.01.030
6. Minkel J, Moreta M, Muto J, et al. Sleep deprivation potentiates HPA axis stress reactivity in healthy adults. Health Psychology. 2014. https://doi.org/10.1037/a0034219
7. Ohayon MM. Epidemiology of insomnia: what we know and what we still need to learn. Sleep Medicine Reviews. 2002. https://doi.org/10.1053/smrv.2002.0186
8. Maimon, Lerner, Admon. Bidirectional interactions between REM sleep and stress responses through the lens of the resilience framework. Translational Psychiatry. 2026. https://www.nature.com/articles/s41398-026-04524-2
9. Qaseem A, Kansagara D, Forciea MA, et al. Management of chronic insomnia disorder in adults: a clinical practice guideline from the American College of Physicians. Annals of Internal Medicine. 2016. https://doi.org/10.7326/M15-2175