You Light Up My Hormones

The Affect of Light on Estrous Cycles

Nancy Kerr

At first glance it seems like female cats randomly go in and out of heat and you wonder if there is rhyme or reason to it. There may be; female cats (often referred to as queens) will go into heat at variable intervals but most typically every 14 to 21 days. Since female cats are seasonally polyestrous and are referred to as long-day breeders (meaning when there are more daylight hours), generally speaking, From late October to January, you may notice that queens do not seem to be cycling as often or may appear to go into a dormant or “rest” period.

Puberty

When a female cat enters puberty, it is based on a number of factors. Cats will generally be between 6 and 12 months of age and at a weight greater than 2-3 kg (roughly 5 to 6.5 pounds, or approximately 80% of adult weight).¹ Coat length also plays a part as shorthair cats tend to reach puberty younger than longhair cats. There are also breed variations as smaller breeds tend to reach puberty earlier than larger breeds. A kitten that is born in the spring and reaches 6 to 8 months of age during the shorter days of the year may not enter puberty until the following breeding season when the amount of day length increases.¹

How Often Does a Female Cat Normally Come Into Heat?

Normal queens begin to have estrous (reproductive) cycles (heats) at 4 to 12 months of age with the average onset at 6 months. The first estrous cycle may occur later in longhaired or larger breeds; these cats may start their first estrous cycle as late as 18 months of age.²

Cats are seasonally polyestrous, which means that they have multiple cycles during the breeding season generally followed by a period of non-cyclicity¹ (anestrus). A queen will cycle during the longer day lengths (more sunlight) and generally will cease when the day length is short (less sunlight). The breeding season will vary according to geographic and environmental factors, such as temperature and the number of daylight hours. In the Northern Hemisphere, cats usually cycle from January until the late fall, then will go into a rest period. However, cats that live in more tropical regions or mainly indoors may cycle all year round.²

How Long Does a Heat Cycle Normally Last?

Each heat generally lasts several days with the average length being six days. If the queen is not mated during estrus, she will go out of heat for a short period of time. Therefore, the complete estrous cycle of a cat can range from anywhere between one to six weeks, with the average cycle length being about three weeks.

Artificial Light and Reproduction

Some queens may exhibit year-round heats due to natural lighting and do not display a seasonal anestrus (period of rest). However, it is difficult to predict which of these cats will continue to come into heat.¹ Temperature may also play a part. If you require to breed your queen during the rest period (late October to January), expose her to 14 hours of artificial lighting for at least two months; doing so will induce and maintain regular heat cycles. If the lighting duration is reduced to eight hours, the heat cycles will cease and the queen will enter an anestrus period.¹ Be aware that the normal household combination of artificial and natural lighting by itself may be not consistent enough to maintain heats year-round.

Seasonality

In the Northern Hemisphere, increasing daylight length in January and February promotes the onset of estrous activity. Peak estrous activity is usually seen in the Northern Hemisphere from February to April. Regular estrous activity will continue until as late as October or November, depending on the geographic distance from the equator (and therefore the length of daylight).³ During the shorter days of winter, indoors cats in North America will generally experience a winter rest period; but the closer to the equator, the less effect of seasonality.

Similar to human females living together, queens housed together may have synchronized heats. Shorthair breeds are less sensitive to the amount of daylight than longhair breeds. Although many longhair queens (such as the Persian breed) will not exhibit regular estrous cycles even during periods of long daylight, many shorthair queens (such as Siamese and related breeds) exhibit estrous cycles year-round, regardless of daylight length.³ Infrequent estrous cycles in indoor cats can be connected to inadequate intensity or duration of light. To encourage regular heats, breeding cats should be exposed to 12 to 14 hours of daylight or artificial light per day.

In Conclusion

The length of exposure to daylight or artificial light does have an effect on feline reproduction. While cats can cycle when the day length is long, they can be induced to cycle year-round with continuous artificial lighting. As a summation:

A period of extended daylight (more than 12 hours per day) following one of short days induces estrus. The longer the short-day period, the more pronounced the reaction. This change from short-day to long-day periods can be successfully repeated at least six times in a year. However, the number of litters produced per year is largest when a regular 12-hour light/dark cycle is maintained.⁴

 

Resources

1. Aime K Johnson, “Normal feline reproduction: The queen,” first published online February 25, 2022, Accessed 01/10/2025, https://journals.sagepub.com/, https://journals.sagepub.com/doi/10.1177/1098612X221079706
2. Ryan Llera, BSc, DVM; Cheryl Yuill, DVM, MSc, CVH, “Estrous Cycles in Cats,” accessed 10/25/2021, https://vcacanada.com/, https://vcacanada.com/know-your-pet/estrus-cycles-in-cats
3. Susan E. Little, “Female Reproduction,” accessed 01/10/2025, https://veteriankey.com/, https://veteriankey.com/female-reproduction/
4. H Hurni, “Daylength and breeding in the domestic cat,” accessed 01/10/2025, https://pubmed.ncbi.nlm.nih.gov/, https://pubmed.ncbi.nlm.nih.gov/7197314/