The Science Behind Daysy
Fertility tracking usually gets
explained with estimates
and assumptions
Let us introduce you to the
biological signal Daysy is built on
and why it's used.
- The biological signal
Yes, it's time for the
science bit
Your body gives you a lot of signals across the month.
Your body gives you a lot of signals across the month.
One of the most reliable is your basal body temperature (BBT) - the temperature you have first thing in the morning before moving, eating, or drinking.
Hormones shift your temperature in a predictable way:
• Before ovulation, estrogen keeps your temperature lower
• After ovulation, progesterone raises it slightly
Daysy reads these small shifts to help you understand where you are in your cycle.
- Temperature across a cycle
What it looks like
Before Ovulation
Lower temperatures
During Ovulation
A transition period as
hormones shift
After Ovulation
Higher temperatures
The changes are very small, often just a few tenths of a
degree, which is why Daysy is built to measure temperature
with medical-grade precision.
The changes are very small, often just a few tenths of a degree, which is why Daysy is built to measure temperature with medical-grade precision.
What Daysy measures
Daysy focuses on your morning temperature
and the pattern it forms over time.
That's enough to classify your fertile
and non-fertile days with 99.4% accuracy.
The algorithm
How Daysy's algorithm works
01
Data Input
Daysy measures your basal body temperature with 0.018 °C precision, collecting accurate readings over time.
02
Pattern analysis
The algorithm analyses how your temperature changes across days, focusing on patterns rather than single readings.
03
Day classification
By identifying post-ovulatory temperature shifts, each day is classified as green (non-fertile), red (fertile), or yellow (still learning).
04
Clinical Validation
This approach is supported by a peer-reviewed study of 5,328 users and over 100,000 cycles, where Daysy identified non-fertile days with 99.4% accuracy
The Stemba clinical view
Clinical perspective on the evidence
Dr Katherine Clark
Clinical Advisor, Stemba
PhD, Women's Health
MSc Clinical Research
MSc Advanced Practice Midwifery
— King's College London
Clinical Research Excellence Fellow
— King's Health Partners, King's College London
Research partnerships with KKH and NUHS, Singapore
That body of evidence is what makes Daysy a useful clinical alternative for women who want cycle awareness without hormones."
-
99.4 % accuracy
-
Clinically validated
-
Hormone free
How it works
60 seconds and
you're done!
Here's the simple version of how it works
01
You take a 60-second temperature reading each morning
02
Daysy updates your pattern automatically
03
It classifies your day as fertile, possibly fertile, or not fertile
04
The lights show you the result - red, yellow, or green
Over time, Daysy learns your unique
rhythm, and your results become
more personalized
- Trusted by 500,000+ women worldwide
Daysy is making their
lives easier
-
I did a lot of research before choosing Daysy and was still a little concerned at first. After using it for a year, I fully trust it and love that I don't have to take hormones. It has proven to be reliable and worth the decision.
Baylee
Verified buyer
-
Give yourself the time of day to learn the tool — it's just a small commitment (1–2 min) every morning and the insights are life changing. Far beyond a period tracker — understand when you've ovulated, backed by science.
Amy Jo S.
Verified buyer
-
Back then I started with zero idea about my cycle and cycle tracking — now even my gynaecologist is always surprised by what I know about my cycle health. My predictions are always correct and the fertile window narrows them down correctly.
Juli
Verified buyer
- Why temperature matters
Why temperature?
But your body isn't average.
Why temperature matters
Why temperature?
Other methods predict fertile windows by estimating based on generic averages of past cycles.
But your body isn't average.
Daysy uses what your body did this morning, not what an app thinks might happen.
This helps create a clearer, less confusing picture over time, especially if:
- Your cycle varies
- You've tried apps that contradict each other
- You want a hormone-free method
- You prefer something simple and quick
- A simple routine
A quick check in the morning, a clearer picture over time
Understanding your cycle shouldn't take over your day.
Daysy is built to be a small, steady routine that becomes clearer with time.
Most people take their reading in under a minute, then get on with their morning.
After a couple of cycles, the "learning days" decrease and the pattern becomes clearer.
A simple routine
A quick check in the morning, a clearer picture over time
Understanding your cycle shouldn't take over your day.
Daysy is built around a small, steady morning habit. Most people take their reading in under a minute, then get on with their day.
After a couple of cycles, yellow days decrease and your cycle picture becomes clearer.
