05/29/2026
Breeding on a Calendar - Article 2: Why Darkness Matters
In Article 1, we talked about the calendar inside the animal.
Not the calendar we write on paper... the biological calendar.
The one influenced by light, darkness, season, nutrition, body condition, hormone signaling, and the animal’s ability to respond.
Now I would like to look more closely at one of the main signals in that calendar:
*Darkness*
That may sound strange at first because most of us hear a lot about light.
- Sunlight.
- Day length.
- Long days.
- Short days.
- Barn lights.
But... in many seasonal breeders, the body is not only measuring how much light it sees. It is also measuring how long darkness lasts.
*This is where melatonin enters the conversation*
- Melatonin is a darkness signal -
Melatonin is often called the “darkness hormone” because the body releases more of it during darkness.
- When nights are short, the melatonin signal is shorter.
- When nights are long, the melatonin signal lasts longer.
Longer darkness signals help tell the body:
The season is changing.
In short-day seasonal breeders, like many sheep and some goats, that message can help shift the reproductive system toward stronger breeding activity.
But melatonin doesn't directly "make an animal pregnant". It also doesn't replace nutrition, body condition, health, semen quality, or timing.
Melatonin is best thought of as a signal. A very important signal, but still just a signal.
- The body is reading night length -
The animal is not thinking:
“It is October. Time to breed.”
The animal is reading environmental cues.
One of those cues is the pattern of light and darkness.
As days shorten and nights lengthen, the melatonin signal changes. That change helps the brain interpret the season.
From there, the brain can influence the hormone system that talks to the ovaries.
The simple version looks like this:
Darkness → melatonin → brain → hormone signaling → ovaries
That is why darkness matters.
- Light can change the message -
If we look at darkness as information, then light at the wrong time can change the information the animal receives.
- It doesn't mean one barn light automatically ruins reproduction.
- It doesn't mean every security light is destroying fertility.
- And it doesn't mean we need to become dramatic about every bulb in the barn (you'll drive yourself crazy if you go this route).
But... it does mean light exposure is not always neutral.
Lights late at night, bright barn lights, show facilities, artificial lighting programs, and inconsistent light exposure can all influence what the animal’s brain interprets as “season.”
- Light programs work because the system can be influenced -
This is why controlled lighting programs can work.
*They are not acting directly on the uterus*
They are attempts to manipulate the animal’s PERCEPTION of season.
Look at it this way... humans can use light and darkness to influence what the brain thinks the season is doing.
- Melatonin products are not fertility in a bottle -
Melatonin may be used in some breeding management systems because it can help mimic or support a seasonal darkness signal.
But... I would not advise going so far as to treat it like a magic switch.
If an animal is thin, sick, stressed, heavily lactating, heat-stressed, parasitized, mineral-deficient, or not biologically ready to respond, melatonin does not erase those problems.
It may help send a seasonal message, but the animal still has to do the biology. A whole lot of things still have to happen correctly to get to pregnancy....
- Follicles still have to develop.
- Hormone signals still have to coordinate.
- Ovulation still has to happen.
- Semen still has to meet the egg at the right time.
- The uterus still has to support early pregnancy.
Melatonin is part of the conversation; it's just not the entire conversation.
- Why this matters for controlled breeding -
Darkness is not magic. It is information.
That is the main point of this article.
- Darkness matters because the body uses it as information.
- Melatonin matters because it helps carry that information.
- Season matters because reproduction is not separate from the environment.
- Light exposure matters because the animal is constantly reading the signals around her, whether we are paying attention to them or not.
Our calendar tells us what day to do something...
But the animal’s body is asking a different question:
What season does this feel like?
That question starts with light.
*And just as importantly, it starts with darkness*
Next in the series:
Article 3 — The Ram and Buck Effect
We will look at how the presence of a male can influence female hormones, behavior, and reproductive activity, and why a ram or buck is more than just a "semen delivery system".
