Biorhythm API: What It Is, How It Works, and How to Use It

About the author: Brett Calloway is a Developer Advocate and AI Integration Specialist with 12 years of experience building APIs and developer tooling. He has led developer relations at two Series B SaaS companies and spoken at PyCon and JSConf on building context-rich AI agents using Model Context Protocol. His writing covers API integration patterns, AI agent architecture, and rapid prototyping with wellness and spiritual data.

What is biorhythm theory

Biorhythm theory proposes that human performance follows predictable sinusoidal cycles starting from the moment of birth. Three primary cycles govern different aspects of daily function: the physical cycle repeats every 23 days, the emotional cycle repeats every 28 days, and the intellectual cycle repeats every 33 days. Each cycle oscillates between a peak (+100%) and a trough (-100%), crossing zero twice per period at what practitioners call critical days.

The mathematical foundation is straightforward. For any given day, you calculate the number of days since birth and apply a sine function: sin(2 * PI * daysSinceBirth / cyclePeriod). The result tells you where in the wave you are. When a cycle is above zero, that faculty is in its "discharge" phase (high performance). Below zero, it enters "recharge" (recovery). The zero crossings are considered unstable transition points where errors and accidents are statistically more likely.

What a biorhythm API does vs a simple calculator

A biorhythm calculator takes a birth date, runs the sine formula, and returns a number. That is roughly five lines of code. A biorhythm API does significantly more, which is why developers choose managed infrastructure over implementing the formula from scratch.

A production-grade biorhythm API provides phase detection (not just the raw value, but whether you are peaking, rising, at a critical crossing, or in recovery), trend analysis (is the value going up or down compared to yesterday), interpretation text (editorial prose explaining what the reading means for that day), and energy ratings (a composite score derived from all three primary cycles).

Beyond single-date readings, a full biorhythm API offers forecasting (multi-day projections with phase transitions highlighted), critical day detection (upcoming zero crossings across all cycles), compatibility scoring (overlay two birth charts and measure alignment), and multi-language support for global applications.

Ready to build this? RoxyAPI Biorhythm API gives you 10 cycle types, 6 endpoints, and 8 languages in one subscription. See pricing.

Why developers use a biorhythm API instead of building one

The sine formula is trivial. The infrastructure around it is not. Here is what you would need to build and maintain yourself if you chose to implement biorhythm calculation from scratch:

Interpretation layer. Raw numbers mean nothing to end users. You need editorial content for every cycle at every phase, refreshed regularly to avoid repetition. RoxyAPI provides unique interpretive text for all 10 cycles across 8 distinct phases, totaling over 80 unique interpretation templates.

Internationalization. If your app serves a global audience, you need translations for phase labels, interpretations, and advice text. The RoxyAPI Biorhythm API supports English, German, Spanish, French, Hindi, Portuguese, Russian, and Turkish out of the box.

Multiple endpoints for different use cases. A reading endpoint covers one day. But dating apps need compatibility scoring. Productivity apps need multi-day forecasts. Health apps need critical day warnings. Building each of these as a separate, well-documented endpoint with proper input validation takes weeks.

Maintained infrastructure. API versioning, rate limiting, caching, documentation, uptime monitoring, and error handling. These are not biorhythm problems, but they consume more engineering time than the core calculation ever will. You also need ongoing maintenance as your user base grows: scaling concerns, cache invalidation strategies, and keeping documentation in sync with actual behavior.

Structured data for AI agents. If your app integrates with LLMs, AI agents, or chatbots, you need well-typed JSON responses with consistent field names and rich descriptions. A biorhythm API designed for AI consumption includes MCP tool definitions, OpenAPI specs, and response schemas optimized for agent reasoning.

10 cycle types that no other biorhythm API covers

Most biorhythm calculators stop at the 3 primary cycles. The RoxyAPI Biorhythm API calculates 10 distinct cycle types organized into three tiers:

Primary cycles (classical biorhythm):

• Physical (23-day period): stamina, strength, coordination, immunity
• Emotional (28-day period): mood, sensitivity, creativity, social awareness
• Intellectual (33-day period): memory, analytical thinking, decision-making, learning capacity

Secondary cycles (extended biorhythm):

• Intuitive (38-day period): gut feelings, subconscious perception
• Aesthetic (43-day period): creative expression, appreciation of beauty
• Awareness (48-day period): self-reflection, mindfulness, perception clarity
• Spiritual (53-day period): inner peace, sense of purpose, connection

Composite cycles (derived from primary pairs):

• Passion (physical + emotional average): romantic energy, drive
• Mastery (physical + intellectual average): skill execution, athletic strategy
• Wisdom (emotional + intellectual average): empathetic reasoning, judgment

Each cycle includes a value from -100 to 100, raw sine value, 8-state phase detection, trend direction, day position within the cycle, days until the next peak, trough, and critical crossing, plus a full interpretation paragraph.

6 endpoints for every biorhythm use case

The Biorhythm API reference documents six endpoints, each designed for a specific integration pattern:

The /reading endpoint is the most comprehensive. Here is a real request and response from production:

curl -X POST https://roxyapi.com/api/v2/biorhythm/reading \
  -H "Content-Type: application/json" \
  -H "X-API-Key: YOUR_API_KEY" \
  -d '{"birthDate":"1990-07-15","targetDate":"2026-04-11"}'

Response (abbreviated to show structure):

{
  "birthDate": "1990-07-15",
  "targetDate": "2026-04-11",
  "daysSinceBirth": 13054,
  "cycles": {
    "physical": {
      "value": -40,
      "rawValue": -0.40,
      "phase": "falling",
      "phaseLabel": "Falling",
      "dayInCycle": 18,
      "daysUntilPeak": 11,
      "daysUntilTrough": 5,
      "daysUntilCritical": 8,
      "trend": "falling",
      "interpretation": "..."
    },
    "emotional": {
      "value": 97,
      "rawValue": 0.97,
      "phase": "peak",
      "phaseLabel": "Peak Performance",
      "dayInCycle": 7,
      "daysUntilPeak": 0,
      "daysUntilTrough": 14,
      "daysUntilCritical": 7,
      "trend": "peaking",
      "interpretation": "..."
    },
    "intellectual": {
      "value": -46,
      "rawValue": -0.46,
      "phase": "falling",
      "phaseLabel": "Falling",
      "dayInCycle": 25,
      "daysUntilPeak": 14,
      "daysUntilTrough": 6,
      "daysUntilCritical": 9,
      "trend": "falling",
      "interpretation": "..."
    }
  },
  "energyRating": 5,
  "overallPhase": "mixed",
  "interpretation": "Your emotional cycle is at peak performance today...",
  "advice": "Lean into social and creative activities...",
  "criticalAlerts": []
}

This response gives you everything needed to render a biorhythm dashboard, power an AI chatbot response, or drive notification logic in a wellness app.

How biorhythm calculation works under the hood

The core formula for any biorhythm cycle is:

value = sin(2 * PI * daysSinceBirth / cyclePeriod) * 100

Where daysSinceBirth is the integer count of days from the birth date to the target date, and cyclePeriod is the fixed length of that cycle (23, 28, 33, 38, 43, 48, or 53 days depending on the cycle type).

Composite cycles average the raw sine values of their two parent cycles before converting to a percentage. For example, the Passion cycle is:

passion = ((sin(2*PI*days/23) + sin(2*PI*days/28)) / 2) * 100

Phase detection goes beyond a simple positive/negative check. The API classifies each reading into one of 8 distinct phases: peak (90-100), high (50-89), rising (10-49), critical ascending (near zero going up), critical descending (near zero going down), falling (-10 to -49), low (-50 to -89), and trough (-90 to -100). This granularity lets apps deliver contextually appropriate messaging rather than a binary "good day / bad day" assessment.

The energy rating condenses the three primary cycle values into a single 1-10 integer score, making it easy for apps to display a quick daily snapshot without overwhelming users with raw data.

Frequently Asked Questions

Q: What is a biorhythm API? A: A biorhythm API is a web service that calculates biorhythm cycle positions from a birth date and target date. It returns structured data including cycle values, phase classifications, trend directions, interpretive text, and energy ratings. Developers integrate biorhythm APIs into wellness apps, dating platforms, productivity tools, and AI agents.

Q: How accurate is biorhythm calculation? A: The mathematical calculation is deterministic and precise. Given a birth date and target date, the sine function produces an exact value. The RoxyAPI Biorhythm API includes a gold standard test suite verifying calculation accuracy. Whether biorhythm cycles correlate with real-world outcomes is a separate question. The API provides the data; interpretation is left to the application context.

Q: Is there a free biorhythm API? A: RoxyAPI offers a Starter plan that includes access to all biorhythm endpoints along with every other domain in the platform. Each plan includes 5,000 to 1,000,000 requests per month depending on the tier. Check the pricing page for current rates.

Q: Can I use a biorhythm API in a dating app? A: Yes. The /compatibility endpoint accepts two birth dates and returns per-cycle alignment scores, an overall compatibility percentage, relationship rating, strengths, challenges, and practical advice. This is specifically designed for dating platforms, couples coaching apps, and relationship wellness tools.

Q: What languages does the biorhythm API support? A: The API supports 8 languages: English, German, Spanish, French, Hindi, Portuguese, Russian, and Turkish. Pass the lang query parameter (e.g., ?lang=es) to receive interpretations, phase labels, and advice text in the specified language.

Build with biorhythm data today

A biorhythm API transforms a simple mathematical formula into production-ready wellness intelligence. With 10 cycle types, 8 phases per cycle, 6 specialized endpoints, and 8 languages, the RoxyAPI Biorhythm API covers every biorhythm integration scenario developers encounter. Whether you are building a wellness dashboard, powering an AI health agent, or adding a compatibility feature to a dating app, structured biorhythm data is one POST request away.

The biorhythm calculation API is part of a single subscription that includes every domain RoxyAPI offers. One API key, one rate limit pool, one set of documentation. Explore the full API reference or check pricing to get started.