SAD Daylight Planner

Introduction: How the Calculator Works

The calculator uses two main steps:

1. Estimate the number of daylight hours for a given month.
2. Translate those daylight hours and your latitude into a suggested number of minutes of bright light per day.

1. Estimating daylight hours

Day length varies throughout the year and depends strongly on latitude. Near the equator, day and night are close to 12 hours year-round. Farther north or south, winter days become shorter and summer days longer. To keep things fast and simple, the planner uses a smooth, repeating (sinusoidal) curve to stand in for these seasonal changes.

The approximate daylight hours D for month m (where January = 1, February = 2, …, December = 12) are modeled as:

In plain language, this formula:

• Starts from a baseline of 12 hours of light per day.
• Adds or subtracts up to about 2 hours based on the time of year.
• Peaks around mid-year (longer days) and dips in the winter months (shorter days).

It does not attempt to model latitude in a physically precise way; instead, latitude is used separately in the exposure estimate. This keeps the model simple and predictable, at the cost of some realism.

2. Converting daylight to suggested bright-light minutes

Once daylight hours are estimated, the calculator suggests a daily bright-light target in minutes, E, using your latitude lat (in degrees):

E = 30 + max(0, 12 − D) × 5 + |lat| / 90 × 10

Each part of this equation serves a purpose:

• Base 30 minutes: A common clinical suggestion for light therapy is at least about 30 minutes of bright light in the morning for many people with winter-pattern SAD.
• Extra time for short days: max(0, 12 − D) × 5 adds 5 extra minutes for every hour that your current day is shorter than 12 hours. When days are very short, the suggested exposure increases.
• Adjustment for latitude: |lat| / 90 × 10 adds up to 10 minutes for people who live near the poles, reflecting generally longer and darker winters at higher latitudes.

The final number is a rounded, educational estimate—not a prescription, dose, or guarantee of benefit.

Worked Example

Suppose you live at latitude 50° (for example, parts of Canada, the UK, or Northern Europe) and want to plan for December, so m = 12.

1. Estimate daylight hours (D). Using the daylight formula, December comes out to roughly 8 daylight hours in this simplified model.

2. Calculate extra minutes for short days. A 12-hour day minus an 8-hour day is 4 hours of “missing” light:

   max(0, 12 − 8) × 5 = 4 × 5 = 20 extra minutes.

3. Account for latitude. At 50° latitude:

   |50| / 90 × 10 ≈ 5.6 extra minutes (the calculator may round this internally).

4. Combine all parts.

   E = 30 + 20 + 50 / 90 × 10 ≈ 30 + 20 + 5.6 ≈ 55–60 minutes

The planner will output a suggested bright-light target of just under an hour per day. You might, for example, aim for 45–60 minutes of morning light, adjusting based on comfort, schedule, and guidance from a professional.

Interpreting Your Result

When you enter your latitude and month, the planner shows a recommended daily bright-light duration in minutes. Here is how to make sense of the number you see:

• Think in ranges, not exact numbers. If the planner suggests 48 minutes, consider it a ballpark range (for example, 30–60 minutes), not a hard requirement.
• Prioritize morning light. Many studies of light therapy for SAD focus on exposure soon after waking, as morning light has a strong effect on your internal clock (circadian rhythm).
• Consider your environment. Spending time outside on a bright overcast day can deliver much more light than ordinary indoor lighting. If you are using a light box, follow the device’s instructions for distance and brightness.
• Increase gradually. If you are new to bright-light exposure, starting with shorter sessions (for example, 15–20 minutes) and building up may feel more comfortable.
• Watch how you feel. Track changes in mood, sleep, and energy. If you notice worsening anxiety, agitation, trouble sleeping, or signs of hypomania/mania (especially if you have bipolar disorder), stop and contact a clinician promptly.

The number you receive is best used as a conversation starter with a healthcare professional or therapist. It can help you explain how dark your winters feel and what kind of light routine you are considering.

Factors That Influence Your Light Needs

People vary widely in how they respond to light. Two people living at the same latitude can have different experiences of winter and different responses to light therapy. Important factors include:

• Indoor versus outdoor light. Outdoor daylight, even on a cloudy day, is usually several times brighter than typical indoor lighting. If you cannot reliably get outside, you may depend more on a lamp or light box.
• Cloud cover and weather. Overcast skies reduce the amount of light you receive compared with clear, sunny days. The calculator does not adjust for local weather, so think about how many truly bright days you have in your location.
• Work schedule. People who commute in the dark and spend the day under dim indoor lights may get far less bright light than those who have flexible daytime schedules, even at the same latitude.
• Window glass and distance. Glass and distance from the light source both reduce intensity. Sitting right beside a window or light box is different from being across the room.
• Sensitivity to light. Some people develop headaches, eye strain, agitation, or sleep disruption with longer bright-light sessions. Others tolerate more time without difficulty. Personal history and medical conditions matter.

Because of all these variables, your ideal light routine may differ from the calculator’s estimate. Consider its output one piece of information alongside professional advice and your own experience.

Sample Daylight Comparison

The table below gives a simplified snapshot of how estimated daylight hours and suggested bright-light time can differ by latitude in January. The values are illustrative and based on the model used in this tool, not precise astronomical calculations.

As latitude increases and winter daylight shrinks, the model suggests gradually longer bright-light exposure. However, clinical recommendations may differ based on individual history, diagnosis, and specific devices used.

Assumptions and Limitations

This tool is intentionally simple and is built on several important assumptions and limitations:

• Simplified daylight model. The sinusoidal daylight formula is a smooth approximation. It does not incorporate precise solar position calculations, time zone effects, or the exact tilt of the Earth for your city. Actual sunrise and sunset times at your location will differ.
• No local environment factors. The calculation does not adjust for weather, air pollution, mountains, buildings, or how much time you actually spend outdoors versus indoors.
• Generalized exposure rule. The exposure formula is a heuristic based on common clinical patterns (for example, ~30 minutes of morning light therapy for winter SAD) but is not based on your medical history, medications, or diagnosis.
• Not a diagnostic tool. The planner does not screen for depression, bipolar disorder, or other mental health conditions. It cannot tell you whether you have SAD or how severe your symptoms are.
• Device differences. Light boxes, dawn simulators, and other devices vary in brightness (lux), wavelength, and recommended distance. The calculator does not take specific product details into account.
• Individual variability. People respond differently to light. Some may benefit from less exposure than suggested here; others might need more, or might not find light therapy helpful at all.

Because of these limitations, the numbers you see from this planner should always be interpreted cautiously and in context. They are best used as an approximate guide and a way to understand how latitude and season influence daylight—not as a substitute for individualized medical recommendations.

Health Disclaimer and Safety Notes

This calculator is for general informational and educational purposes only. It does not provide medical advice, diagnosis, or treatment and is not a replacement for care from a qualified healthcare professional.

Talk with a clinician before starting or changing light therapy if any of the following apply to you:

• You have a history of bipolar disorder, mania, or hypomania.
• You have serious eye conditions (for example, significant retinal disease, severe glaucoma, or recent eye surgery).
• You use medications that increase sensitivity to light (such as some antibiotics, anti-acne drugs, or mood stabilizers).
• You have been advised to avoid strong light exposure for eye, skin, or neurological reasons.
• You experience worsening mood, anxiety, agitation, or insomnia when using bright light.

If you are experiencing thoughts of self-harm, severe depression, or sudden changes in behavior, contact local emergency services or a crisis hotline immediately rather than relying on a calculator or self-help approach.

Frequently Asked Questions

How accurate is this SAD daylight estimate?

The daylight and exposure values are approximate and based on a simplified mathematical model. They can differ from actual sunrise and sunset times or from professional light-therapy plans, and should be treated as rough guidance only.

Can this replace professional treatment for Seasonal Affective Disorder?

No. The planner cannot diagnose SAD or recommend a full treatment plan. If your symptoms are persistent, worsening, or interfere with your daily life, you should speak with a doctor, psychiatrist, or other mental health professional.

Is artificial bright light the same as sunlight?

Bright light boxes can mimic some of the effects of sunlight on your body clock, but they are not identical to natural daylight. Device brightness, spectrum, and duration matter, and you should follow the manufacturer’s instructions and your clinician’s guidance.

How to use: When during the day should I use the suggested bright-light time?

For winter-pattern SAD, many experts focus on morning light soon after waking, which tends to support a more stable sleep-wake rhythm. Evening bright light can sometimes interfere with sleep for some people.

What if the suggested minutes feel like too much for me?

If the estimate seems high, you can start with a shorter duration, notice how you feel, and discuss adjustments with a healthcare professional. Comfort, safety, and overall well-being are more important than matching the exact number from the planner.

Background and Sources

This tool is loosely informed by widely used clinical practices for light therapy in seasonal depression, where morning bright light is often recommended at strengths of around 2,500–10,000 lux for set periods of time. Authoritative organizations and review articles describe light therapy as one option among several for managing SAD, alongside approaches such as psychotherapy, medication, sleep hygiene, and lifestyle changes.

Because guidelines evolve over time, always rely on up-to-date recommendations from trusted health organizations and licensed professionals rather than this calculator alone. Use the suggested minutes as a starting point for conversation and planning, not as a definitive treatment dose.