In a few weeks, the clocks will fall back an hour. While your phone may update itself, your body clock doesn’t get the memo. That internal clock runs on light, not on a strict, 24-hour schedule. Researchers who tracked the sleep of 55,000 people found that sleep timing follows the seasonal progression of dawn, not clock changes. The American Academy of Sleep Medicine takes the same view: standard time aligns best with human circadian biology, and the seasonal switches back and forth are associated with health and safety risks.
Here's the honest part: falling back in autumn is usually the more gentle shift. Most people adjust to it more easily than to the spring change. But it also marks the beginning of the darkest stretch of the year. Days keep shortening until late December, evenings go dark around dinner, and more of the day happens under indoor light.
The good news? You have about three weeks to make a few changes that can show up in your cortisol data.
What is your cortisol awakening response?
Your cortisol awakening response, or CAR, is the rise in cortisol that happens in the first 30 to 45 minutes after you wake up. It’s a part of your broader daily rhythm, where cortisol tends to run higher in the morning and fall by nighttime. If you're new to it, start with our guide to what the cortisol awakening response tells you.
Two things worth knowing up front:
The size of the rise varies. One review of the research reported increases of roughly 50% to 160% in the first 30 minutes after waking, and noted there are no agreed norms for absolute morning levels. That's why your own baseline matters more than anyone else's number.
CAR is sensitive. Researchers list light, wake time, and how closely people stick to their testing schedule as key factors that shape it. That sensitivity is exactly what makes it interesting to track, and it's why timing matters when you test.
Why light is a part of the conversation.
Your body's master clock sits in the brain and takes its main timing cue from light hitting your eyes. Morning light is the strongest signal indicating "it's daytime now.” We cover the bigger picture in understanding your body's clock.
Several studies have looked at light and morning cortisol specifically:
• In a study of 14 healthy men, an hour of 800 lux light after waking was associated with cortisol levels about 35% higher than waking in darkness. The same light in the evening had no effect.
• Waking to a dawn simulation light, which brightens gradually before your alarm, was associated with higher cortisol at waking compared to waking in the dark.
• A 2023 systematic review concluded that bright light in the late night or early morning was associated with higher cortisol than time-matched dim light.
Now, some caveats to consider since not every study arrives at the same conclusion. Light has been reported to raise, lower, or have little effect on cortisol, depending on how bright it was, how long it lasted, and when it happened. Most of these studies were also modestly sized and run in labs.
So while it’s impossible to promise a bigger or steeper CAR, the research supports that morning light is a real factor that shapes your cortisol, and that people respond differently. That in of itself is a a good reason to test it on yourself and gather your own data.
The three week light experiment you can try.
Start the week of October 12th and you'll finish right before Nov 1st.
Week 1: Map your cortisol baseline. On 2–3 mornings, capture your CAR with three tests: one right when you wake, one at +30 minutes, and one at +60 minutes. Don't change anything else yet.
Weeks 2–3: Add light. Right after your first morning test, get 10–15 minutes of outdoor light. Sit on the porch, stand on the balcony, or open the door and step out. On gray days, outdoor light is still usually much brighter than indoor lighting. If outside isn't an option, sit next to your brightest window. When possible, skip sunglasses, but never look directly at the sun.
End of week 3: Retest. Run the same three-test protocol as Week 1 across 2–3 mornings and compare your rise from waking to peak against Week 1.
A few tips to help guide your readings:
Avoid heavy exercise between tests. Light is the only change you're making, so skip the workout until after your +60 minute test. If you're curious about movement, read which type of exercise is best for your cortisol and test it separately later.
Hold the coffee until after your last test. Caffeine can affect cortisol, so it muddies the comparison. Here's what coffee does to your daily rhythm.
Keep your wake time steady. Aim for roughly the same wake time across all three weeks (weekends included.)
Test right after waking up. Your first test should happen right as you get up. Our guide on when to test cortisol walks through timing.
How to understand your cortisol reading.
Look at the shape of your rhythm, not a single reading. Compare the rise from your waking test to your highest reading, and compare averages across mornings rather than one day against another.
Expect some noise. CAR shifts day to day with sleep, stress, and your schedule. If one morning looks off, see why your cortisol reading might look different than expected before drawing conclusions. (The Tag Trends feature in the Eli app is helpful here.)
If your rise looks bigger after the weeks with more light exposure, that's a signal your mornings may respond to light. If nothing changes, that's useful too. It tells you light may not be the lever for you, and you can test something else next.
What to do after Daylight Saving.
When the clocks change, keep your light habit anchored to your wake time on the new clock. The habit stays the same; only the number on the clock moves. If your schedule gets thrown off, our post on realigning your rhythm after jet lag covers the same principles at a bigger scale. And if you want to go deeper on what you can and can't change, read more here on what can actually shift your cortisol rhythm.
Hormometer® is a wellness tool for tracking your cortisol rhythm, not a diagnostic device. If stress is significantly affecting your daily life, that's worth a conversation with your physician or healthcare provider.
