How Daysy Works: The Science of Reading Your Body

Many people mistake Daysy for a common digital thermometer, but the difference is significant. Daysy is a Class I medical device that builds on the calculo-thermal method and combines it with a self-learning algorithm; what we call the Fertility Tracker Method.

The clinically validated 99.4% accuracy of Daysy ³ comes from the combination of two factors:

• Your real data. Every day, in about 60 seconds, Daysy measures your basal body temperature (your body temperature at rest, taken sublingually first thing in the morning) and records the start of your period.

• Collective intelligence. Daysy's algorithm has been continuously developed since 1986, refined across more than 40 years and over 10 million analyzed cycles.

When you place the high-precision sensor (0.05 °C precision) under your tongue, the device doesn't simply record a number. It analyzes your temperature curve, compares your individual pattern against decades of cycle data, and identifies exactly where you are in your cycle.

The Daysy algorithm does three things:

1. It recognizes the temperature shift that marks ovulation, and distinguishes biphasic ovulatory cycles ³ from anovulatory ones.

2. It adapts to your individual cycle, whether long, short, regular, or irregular.

3. It applies fertility-awareness rules to your temperature data. This is not raw tracking; it is an automated application of evidence-based evaluation rules to your daily measurements.

The algorithm works conservatively. It builds in buffer days. As long as it is not clearly certain that a phase is non-fertile, Daysy does not show green; it shows red or yellow. In practice, this means Daysy displays more fertile days, not fewer, than a manual temperature method alone would ³ . That is intentional, and it is the opposite of what some critics suggest.

Daysy translates the complexity of biology into an intuitive light system. It is your daily orientation:

• Red light (potentially fertile/ fertile). Daysy identifies that you are inside your fertile window or that ovulation is approaching.

• Green light (non-fertile). Daysy has identified this day as non-fertile based on your data and the algorithm's evaluation.

• Yellow light (learning or fluctuation). You will see this light more often at the start, while Daysy gets to know your cycle, or when there is a significant variation in your data. When in doubt, the algorithm stays on the cautious side.

Daysy does not try to predict your cycle based on averages from other women, the way many free calendar apps do. It learns from you. If your ovulation is delayed because of an illness or a stressful phase, Daysy detects that thermal variation and adjusts the lights accordingly.

A common question about calculo-thermal devices: how can they work without cervical mucus, a key marker in the manual symptothermal method?

The answer lies in intelligent risk management and in the limits of mucus observation itself.

Cervical mucus assessment is highly subjective and depends on individual ability to recognize and interpret changes in consistency and quality. Misinterpretations are common, and the mucus-only method has a steep learning curve.

Daysy compensates for this in two ways. First, the algorithm opens the fertile window (red light) early, based on your earliest likely ovulation day, with a spermatozoa-survival buffer built in ². Second, the data shows that very early ovulation, where mucus observation would matter most, is extraordinarily rare: in the Soumpasis 2020 analysis of 75,981 cycles, no ovulation was observed before cycle day 8 ⁴. And even when very early ovulation does occur, the successful pregnancy rate in those cycles is only 9.3%; ¹ the body's own biology acts as a safeguard, because an early-ovulation cycle often leaves the uterine lining too thin for implantation. Daysy's rule structure operates comfortably inside this safety zone.

Cervical mucus can still be documented in the DaysyDay App for your own observation. It simply does not enter the algorithm, because the algorithm does not need a subjective input to do its job reliably.

Choosing Daysy is a one-time investment in your cycle literacy. Unlike solutions that tie you to monthly fees just to access your own data, Daysy offers full ownership: your device, your data, your DaysyDay App, your Partner App, all without a subscription.

Your body speaks. Daysy translates.