SAD Daylight Planner

How the SAD Daylight Planner turns month and latitude into minutes

The SAD Daylight Planner works in two steps:

1. Estimate the seasonal daylight pattern for the month you choose.
2. Translate that estimate plus your latitude into a suggested number of bright-light minutes per day.

1. Estimating daylight for your month

In this planner, the month is used to trace a smooth seasonal daylight curve. That keeps the calculation quick and easy to read while still letting winter and summer behave differently.

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 and dips in the winter months.

The daylight curve is intentionally simple. Latitude is not folded into this first step; instead, it enters later when the calculator turns daylight into a minutes target. That keeps the planner easy to follow, even though it is less detailed than a city-specific sunrise and sunset table.

2. Turning daylight into a suggested bright-light target

Once the daylight estimate is in place, the calculator converts it into 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 has a specific role:

• Base 30 minutes: The planner begins from a common light-therapy starting point of about half an hour of bright morning light.
• Extra time for shorter days: max(0, 12 − D) × 5 adds five minutes for every hour the modeled day falls below 12 hours. The shorter the day in the seasonal curve, the larger this adjustment becomes.
• Latitude adjustment: |lat| / 90 × 10 adds up to 10 minutes for locations near the poles, where winter darkness is usually more pronounced.

The final number is a rounded planning estimate, not a medical dose and not a promise that more minutes will always be better.

Worked Example: Planning December light exposure at 50° latitude

This SAD Daylight Planner example uses latitude 50° and December to show how the month-based daylight estimate becomes a minutes target.

1. Estimate daylight hours (D). For month 12, the seasonal curve gives:

   D = 12 + 2 × sin((2π × (12 − 3)) / 12) = 10

   So the planner treats December as a 10-hour day in this simplified model.

2. Calculate the short-day adjustment. A 12-hour reference day minus a 10-hour modeled day leaves 2 hours to account for:

   max(0, 12 − 10) × 5 = 2 × 5 = 10 extra minutes.

3. Account for latitude. At 50° latitude:

   |50| / 90 × 10 ≈ 5.6 extra minutes.

4. Combine the pieces.

   E = 30 + 10 + 5.6 ≈ 45.6

   The calculator rounds that to 46 minutes per day.

So in this SAD Daylight Planner example, a December day at 50° latitude lands at just under three-quarters of an hour of bright light. That is best treated as a flexible target you can discuss and adjust with a clinician if needed.

How to read the SAD Daylight Planner result

When the SAD Daylight Planner gives you a number, think of it as a starting range rather than a strict rule. The output is meant to help you compare seasons and latitudes, not to tell you exactly how long you must sit in the sun or in front of a light box.

• Use ranges, not a single perfect minute count. If the planner suggests 43 minutes, a practical routine might still live somewhere around that mark depending on your schedule and how you feel.
• Morning is the usual starting point. Many SAD light routines begin soon after waking because morning exposure is less likely to disrupt sleep than late-day bright light.
• Indoor and outdoor light are very different. A cloudy walk outside can still be far brighter than normal room lighting, while a light box depends on device brightness and distance.
• Start gradually if you are new to light therapy. Shorter sessions can be easier to tolerate while you learn what feels comfortable.
• Pay attention to your body. Track sleep, mood, agitation, and eye comfort. If brighter light makes you feel wired, anxious, or unable to sleep, stop and check in with a professional.

The number is most useful when it helps you plan a realistic routine and gives you a concrete way to talk about winter light exposure with a healthcare provider or therapist.

What can push your SAD light needs up or down

The SAD Daylight Planner cannot see every detail of your routine, so the number it gives you is only part of the picture. Two people with the same latitude and month can still need different light habits because of daily life, weather, and medical history.

• Outdoor time versus indoor time. A short walk in daylight may deliver far more light than an hour spent under ordinary office lamps.
• Cloud cover and weather. Gray skies, snow glare, rain, and atmospheric haze all change how bright the day feels, but the calculator does not try to model local weather.
• Schedule and commute. If you travel to work before sunrise or spend most of the day in dim indoor spaces, your real light exposure may be lower than the planner implies.
• Windows and distance. Glass, angle, and how far you are from a light source can change intensity a lot, so the same number of minutes can feel very different in practice.
• Personal sensitivity. Some people feel calm and energized with bright light, while others get headaches, eye strain, or sleep disruption quickly.

Because of those differences, the planner should be read as one useful clue, not as the final word on your ideal light routine.

Sample SAD Daylight Planner outputs

For this sample, the planner keeps the month fixed at January so you can see how the suggested minutes rise as latitude moves farther from the equator. The daylight estimate stays tied to the month, while the recommended bright-light time changes with latitude.

That pattern reflects the formula in this SAD Daylight Planner: the month sets the seasonal daylight curve, and latitude adds extra minutes when you live farther from the equator.

Assumptions and Limits of the SAD Daylight Planner

The SAD Daylight Planner deliberately uses a simple model, so it comes with several assumptions and limits you should keep in mind:

• Simplified daylight curve. The sinusoidal month-based formula smooths out the year into an easy-to-read wave. It does not calculate sunrise and sunset for a specific city or account for time zone quirks.
• Latitude used only once. In this calculator, latitude affects the suggested minutes, not the daylight estimate itself. That keeps the logic straightforward, but it is less detailed than a location-specific astronomical model.
• No weather or setting adjustments. The calculation does not know whether you are outdoors, sitting by a window, living under heavy cloud cover, or using a light box at a particular distance.
• Generalized exposure rule. The minutes target is a rule of thumb built around common light-therapy starting points. It does not account for your diagnosis, medications, or history.
• Not a screening tool. The planner cannot tell you whether you have SAD, whether your symptoms are seasonal, or which treatment is safest for you.
• Personal response varies. Some people do well with the suggested range, while others need less time, more time, or a different approach altogether.

For those reasons, treat the output as an approximate guide. It is helpful for comparing months and latitudes, but it should not replace individualized medical advice or a properly designed light-therapy plan.

Health Disclaimer and Safety Notes

This SAD Daylight Planner is for general informational and educational use only. It does not diagnose Seasonal Affective Disorder, prescribe treatment, or replace 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.

SAD Daylight Planner FAQs

How close is the SAD Daylight Planner to real-world daylight planning?

It is a rough planning aid. The month-based daylight curve and the latitude adjustment are intentionally simple, so the result is best used as a starting point for planning and discussion rather than a precise light-therapy dose.

Can the SAD Daylight Planner replace professional treatment?

No. It cannot diagnose Seasonal Affective Disorder or build a treatment plan for you. If your symptoms are persistent, worsening, or interfering with daily life, talk with a doctor, psychiatrist, therapist, or other qualified clinician.

Does the planner treat sunlight and a light box the same way?

No. Natural daylight, cloudy outdoor light, and a light box can feel very different. The calculator only gives a minutes target and does not measure lux, distance, or the brightness of any specific device, so product instructions and professional guidance still matter.

When should I use the suggested SAD light minutes?

For winter-pattern SAD, many people start with morning light soon after waking because it is less likely to disturb sleep. If morning light feels too activating, ask a clinician how to adjust the timing safely.

What if the suggested number feels too high for me?

Think of the result as a ceiling, not a requirement. Starting with a shorter session, noticing how your mood and sleep respond, and adjusting slowly with professional advice is often more realistic than trying to match the number exactly on day one.

Background and Sources

The SAD Daylight Planner is loosely informed by common light-therapy practice for seasonal depression, where morning bright light is often used in sessions measured in minutes. Clinical reviews and health organizations generally describe light therapy as one option among several, alongside psychotherapy, medication, sleep hygiene, and lifestyle changes.

Because guidance evolves and people respond differently, rely on up-to-date recommendations from trusted health organizations and licensed professionals rather than this calculator alone. Use the suggested minutes as a planning prompt, not as a definitive treatment dose.