Circadian Rhythm Calculator: Map Your Body Clock and Light Timing

Your body clock runs a daily program. Melatonin starts rising about 2 to 3 hours before your usual bedtime ^[1], your core temperature hits its low point a few hours before you wake ^[2], and light on either side of that low point nudges the whole program later or earlier ^[4]. Enter when you fall asleep and wake up on free days, and this calculator turns those research averages into clock times for you. Sleep from midnight to 8:00 AM, and you can expect melatonin to start rising around 9:30 PM and your temperature to bottom out around 4:30 AM.

How the circadian rhythm calculator works

A lab can measure your clock directly by collecting saliva every half hour in dim light, catching the moment melatonin switches on. Most of us will never do that, so the calculator works backward from your sleep. It asks about free days on purpose: in one study, melatonin timing tracked sleep times much more closely in people who slept when they liked than in people on fixed schedules ^[9]. If an alarm decides your mornings, your workday times say more about your job than about your clock.

The research gives ranges, and we had to pick a point inside each one. These are our own rules of thumb, built on the studies cited, not a validated clinical formula:

• Melatonin onset = time you fall asleep - 2 h 30 min, the middle of the usual 2 to 3 hours ^[1]
• Temperature low point = wake-up time - 3 h 30 min, inside the usual 3 to 4.5 hours ^[2]
• Dim-light start = time you fall asleep - 3 h ^[5]
• Evening second wind = 1 to 3 hours before you usually fall asleep ^[6]
• Afternoon dip = 9 to 10 hours after the temperature low point ^[6]

Your melatonin onset

Melatonin is the hormone that tells your body it is biological night. Its evening rise, measured in dim light and called dim-light melatonin onset (DLMO), is the standard marker of where your clock sits. It usually begins 2 to 3 hours before habitual bedtime, and it varies a lot from person to person: in 170 healthy adults it ranged from 6:13 PM to 12:26 AM, and earlier risers tended to have earlier onsets ^[1]. With a spread that wide, a one-size rule like "melatonin rises at 9 PM" is bound to be wrong for many people.

Light can suppress melatonin, even ordinary indoor light ^[1], so the calculator also gives you a time to dim the lights. An international expert panel recommends keeping evening light low, at most 10 lux of melanopic light (a measure weighted for the eye's clock-setting cells) at eye level, from at least 3 hours before bedtime, and keeping the bedroom darker still, below 1 lux, while you sleep ^[5]. In practice that means warm, low lamps instead of bright overhead lights. Our guide to blue light and sleep covers screens.

The temperature low point and why it matters

Your core body temperature drifts down through the night and reaches its minimum a few hours before you wake: with an 8-hour sleep, typically 3 to 4.5 hours before wake time ^[2]. In a study of 172 young adults, the low point averaged about 3:50 AM for morning types, 5:00 AM for people in between and 6:00 AM for evening types, and it sat closer to waking in the later types ^[3]. Waking almost on top of it is one reason an early alarm can feel brutal for a night owl.

The low point matters most as a hinge for light. In a careful lab study, bright light given before the temperature minimum shifted the clock later, light given after it shifted the clock earlier, and light right at it did nothing much ^[4]. Your bedroom temperature is a separate question, covered in the best room temperature for sleep.

Light that moves your clock earlier or later

Pick a goal in the calculator and it turns that hinge into a light plan. To shift earlier, get bright light, ideally daylight, in the first 3 hours after you wake, which falls after your temperature low point, and keep your evenings dim; light late in the sleep period and in the morning after the low point produces advances ^[2]. To shift later, flip it: brighter light in the 3 hours before your usual sleep time and low light for the first 2 hours after waking, because light in the evening and early night, before the low point, produces delays ^[2][4]. To stay put, get plenty of daylight through your day (the expert panel suggests at least 250 lux of melanopic light indoors) and dim light in the last 3 hours before bed ^[5].

The 3-hour and 2-hour windows are our practical choices, not exact lab boundaries. Natural light is a strong signal, though. When eight people spent a summer week camping with only sunlight and campfires, their internal clocks ran about 2 hours earlier, with melatonin onset near sunset ^[7]. For how long to stay outside in the morning, see how long to get morning sunlight.

A worked example

Say you fall asleep at midnight and wake at 8:00 AM on days off. The calculator gives:

The afternoon dip and the evening second wind

Ever hit a wall at 2 PM, then feel oddly alert at 10 PM when you meant to be winding down? The clock is behind both. When researchers studied people living without any clues to the time of day, sleep onsets clustered at the temperature low point and again 9 to 10 hours later. They also found a zone, which in everyday life would fall about 1 to 3 hours before habitual bedtime, when people rarely fell asleep ^[6]. The calculator places your dip and your second wind using those gaps. The afternoon dip shows up even without lunch, though a high-carbohydrate meal makes it worse ^[8]; why you get sleepy in the afternoon explains the rest.

Limits of estimating your clock without a lab test

Treat every time on this page as an estimate with an hour or more of give. For comparison, a regression formula the researchers built specifically for the job, using wake times, came within 1.5 hours of measured melatonin onset in 96% of a small group of free sleepers ^[9]. Our simpler rule of thumb is unlikely to do better. The estimate also gets weaker when your sleep is shaped by an alarm, work or family duties ^[9], and it doesn't work for rotating shifts, because there is no stable rhythm to read from sleep times that change every week.

If you do try to move your clock, go gradually. Light works over days, not in one evening, so keep your wake time steady while you shift. And if for weeks you can't fall asleep or wake at the times your life needs, or you are sleepy enough during the day to be unsafe, see a sleep doctor, who can check for a circadian rhythm sleep disorder.