The stress response and sleep regulate each other, and sleep loss pushes the loop in a direction that makes both worse. The hypothalamic-pituitary-adrenal (HPA) axis is the body's main stress-response system: the hypothalamus releases corticotropin-releasing hormone, the pituitary releases adrenocorticotropic hormone, and the adrenal cortex releases cortisol, which mobilizes glucose and sharpens alertness. Cortisol normally follows a strong daily rhythm, peaking shortly after waking and falling to a low point around midnight.
Under sleep restriction, this rhythm changes in two ways. The evening nadir rises, so cortisol stays higher when it should be low, and the morning rise occurs earlier and is often steeper. The overall daily profile flattens. Higher evening cortisol is itself arousing and interferes with sleep onset, which is the first arm of the loop: sleep loss raises cortisol, and raised cortisol makes the next night's sleep harder to initiate and maintain.
The second arm runs through emotional and cognitive load. A night of poor sleep increases amygdala reactivity and reduces prefrontal control, as the previous page described, so everyday stressors produce larger stress responses. Those responses activate the HPA axis again, adding another cortisol pulse and further fragmenting sleep. Over time the loop can become self-sustaining: fragmented sleep, heightened stress reactivity, more arousal at night, more fragmented sleep.
The mental-health evidence fits this loop. Large epidemiological studies find that people with chronic short sleep or insomnia carry a substantially higher risk of depression and anxiety, and prospective studies show that insomnia often precedes a first depressive episode. The direction of the loop is the key point: sleep disturbance is not merely a symptom of mood disorders but also a risk factor that can precede them. Two cautions are important. First, most of this evidence is observational, so it establishes association and temporal precedence rather than proof that short sleep alone causes depression. Second, the relationship is probabilistic: many short sleepers never develop a mood disorder, and many people with depression have no history of sleep problems. What the evidence supports is that chronic sleep loss is a modifiable contributor to mental-health risk, not a sufficient cause.