It could have started 12 hours earlier, when you skipped that walk and stayed in the chair.
We often blame the night for our poor sleep: screens, caffeine, and noise. But through working with Trillies+, using causal modeling, we’re seeing patterns suggesting that many of the biggest drivers of sleep quality live in the previous twelve hours, especially your movement.
Sedentary Days → Flatter Evening Cortisol and Lighter Sleep
Cortisol, the body’s primary stress hormone, naturally peaks in the morning to wake you up and should gradually decline throughout the day, reaching its lowest point at night to allow for restorative rest. On inactive days, however, that natural decline may be less pronounced.
This leaves you in a state of subtle physiological arousal, even if you feel mentally spent from a long day of meetings. Your body is still slightly "on," which can disrupt the transition into the deeper, more restorative stages of sleep. The result is a lighter, less efficient night of rest.
"Tired but Wired" = High Cognitive Load + Low Physical Fatigue
That feeling of being "tired but wired" is a common symptom of the modern sedentary lifestyle. It is the result of a mismatch between two types of fatigue: high cognitive fatigue and low physical fatigue.
When your mind is overstimulated by a heavy mental load, a constant stream of information, and screen time, it can feel exhausted. However, if your body has not accumulated enough "sleep drive" through movement, it is not primed for deep rest. This combination, a restless body and an overstimulated mind, breeds tossing, turning, and fragmented sleep.
Late-Night Heart Rate Spikes? It's Not Always Just Stress.
A healthy night of sleep typically shows a heart-rate dip as the parasympathetic nervous system, or "rest and digest" mode, takes over. Many people who use sleep trackers are concerned by a resting heart rate that doesn't show this dip, and while psychological stress is a factor, it can also be a symptom of a sedentary lifestyle.
Inactivity is linked to lower Heart Rate Variability (HRV) and less parasympathetic dominance. A sedentary day can prevent your body from fully transitioning into a rest state, which can keep your heart rate elevated and fragment your sleep architecture. (It’s important to note that other factors like illness, alcohol, late heavy meals, heat, or pain can also cause a lack of heart rate dip.)
The Takeaway
Sleep isn’t only a bedtime problem. It’s a daytime movement problem, too. By understanding these connections, you can reframe your approach to sleep. Even simple, regular activity, such as a lunchtime walk, light breaks from sitting, or an earlier workout, can build healthy sleep pressure, support a healthier cortisol slope, and improve autonomic balance, setting the stage for a deeper, steadier night of sleep.
With Causal AI, we’re starting to map how daytime choices can shape nighttime sleep patterns, one more step toward understanding the "why" behind the numbers on your wearable.
*If you have persistent insomnia, loud snoring with pauses, or severe daytime sleepiness, please speak with a clinician.
About the Author
Darren S. is the founder of Trillie Inc., bringing over 20 years of experience in technology and product management. He is passionate about advancing causal AI and building sustainable solutions that make practical, affordable technology accessible to people and communities everywhere. Connect on LinkedIn