Seed Cycling (also dubbed as "seed rotation") is an alternative medicine practice that has lately won great favour as a natural way of boosting hormonal health and menstrual well-being. Conditions ranging from PMS to PCOS and even Menopause are believed to benefit from this rising trend, as people look for alternatives to birth control pills and medications.
We first came across the practice of seed cycling in a book on fixing periods and we were immediately intrigued. It is the practice of rotating between two different sets of seeds during the two main phases of the menstrual cycle. Set A (Pumpkin & Flax seeds) is meant to be consumed during the Follicular Phase and Set B (Sesame + Sunflower seeds) is reserved for the second phase, also known as the Luteal phase.
The key principle behind seed cycling is grounded in the idea that "food is medicine". Our diet has been shown to have a significant association with hormonal health.  Specific seeds with their varying compositions of lignan, zinc, vitamin E etc. are believed to provide the right support to the body at the right time for balancing estrogen and progesterone levels.
In order to understand the practice of seed cycling, let's look at a standard 28-day menstrual cycle (this is just an approximate and your cycle can be longer or shorter than this). Each cycle begins with the onset of your period (Day 1 of menstruation) and goes on until the start of your next period (~28 days later).
This is the first phase of a menstrual cycle, beginning from the start of a period up until ovulation (Day 1 - Day 14). During this phase, the body is getting prepared for ovulation and the production of estrogen increases. Seed cycling, during this phase, requires us to consume one tbsp each of pumpkin seeds and freshly ground flax seeds, daily. Pumpkin seeds and flax seeds, being rich in lignan, zinc and omega-3 fatty acids, are thought to bolster estrogen production, clear up excess estrogen and prepare the body for progesterone production for the Luteal Phase.
The second phase of a menstrual cycle is the Luteal Phase and it begins from ovulation up until the start of your next period (Day 15 - Day 28). This phase is characterized by the thickening of the uterine lining as the body prepares itself for implantation and pregnancy. The key hormone needed during this phase is progesterone.
Seed cycling, for this phase of the cycle, depends upon the consumption of sunflower seeds and sesame seeds (one tbsp each, daily). Sesame and sunflower seeds are rich in zinc, selenium and fatty acids which support progesterone production. Sunflower seeds are also rich in Vitamin E and selenium which possess progesterone properties and display antioxidant activity beneficial for reproduction. 
If you are going through menopause or are not cycling, you could start with seed cycling at any time, alternative between the two sets every two weeks. Delicious ways of incorporating seeds in your diet is to mix it up with smoothie bowls, salads and energy balls. Our favorite recipes for seed mixes can be found here.
Unfortunately, no one can say for sure. The heavy reliance on phrases such as "believed to", coupled with the lack of citations is a good indicator of the paucity of scientific evidence surrounding seed cycling. We find health coaches, bloggers and experts to be split on this naturopathic practice, with some refusing to entertain the concept due to lack of evidence and others recommending it as a period fix-all.
It may also be useful to note that while, there isn't enough data around seed rotation, there have been some small studies conducted on the individual effects of the four seeds on hormonal health. For instance, a study done in 1993 on the 'Effect of Flax Seed Ingestion on the Menstrual Cycle', shows that for women consuming a western omnivorous, low fiber diet, adding flax was consistently associated with lower chances of anovulatory cycles (possible benefit for fertility) and increased Luteal Phase lengths . Similarly, for those with PCOS, supplementing with flax has also been suggested for reducing androgen levels. 
Zinc (present in pumpkin and sesame seeds) has been linked to preventing menstrual cramps by inhibiting the metabolism of prostaglandins. While, a study done in 1972 has shown Vitamin E (present in sunflower seeds) to possess oestrogenic, androgenic and progesterone-like properties. Vitamin E also demonstrates antioxidant activity against reproductive disorders.  We'd be remiss, however, to not mention that many of these studies have either been of small sample sizes, conducted two or three decades ago or produced statistically insignificant results.
The bottom line, therefore, In our view is that while, seed cycling may not be the 80/20 to improved hormonal health, there doesn't seem to be any major risks to incorporating this practice into your routine. In other words, it can be worth a try. You can also always choose to include the seeds in your diet without conforming to a specific rotation, since there is some evidence, albeit limited in nature, around the benefits of individual seeds for menstrual health.
The Girlmantra team has also personally given seed cycling a go in the last couple of months, only to find mixed results. While two out of five of us reported feeling better with less severe cramps, the remaining three of us could not point out any major differences. We used the seeds across a few different recipes with some of us just eating it straight by the spoonful. The differences in results could be attributed to different diets, physical activity levels or even just placebo. Either way, it has made us more conscious of incorporating more seeds in our diet and we feel better for it!