• Infradian rhythms including the menstrual cycle and seasonal affective disorder and therapies, including light therapy. 
  • Research into infradian rhythms.

The Menstrual Cycle

The menstrual cycle is the series of physiological changes that occur in a woman’s body in preparation for the possibility of pregnancy. The menstrual cycle is controlled by the interaction of hormones produced by the hypothalamus, pituitary gland, and ovaries. The average menstrual cycle lasts 28 days, but it can range from 21 to 35 days.

The menstrual cycle can be divided into three phases: the menstrual phase, the follicular phase, and the luteal phase.

  1. The menstrual phase starts on the first day of menstruation, during which the lining of the uterus is shed through the vagina. This phase typically lasts for 3-7 days.
  2. The follicular phase starts after the menstrual phase and lasts for about 14 days. During this phase, the pituitary gland releases follicle-stimulating hormone (FSH), which stimulates the growth of a follicle in the ovary. The follicle contains an egg. As the follicle matures, it releases estrogen, which causes the uterus to build up its lining in preparation for pregnancy.
  3. The luteal phase begins after ovulation, which is when the egg is released from the ovary. During this phase, the ovary releases progesterone, which thickens the lining of the uterus in preparation for pregnancy. If the egg is fertilized and implants in the uterus, the fertilized egg produces the hormone human chorionic gonadotropin (hCG), which maintains the thickened lining of the uterus. If pregnancy does not occur, the thickened lining is shed, and the cycle starts again with the menstrual phase.

The menstrual cycle is regulated by a complex interplay of hormones, and any disruptions in this delicate balance can lead to menstrual irregularities. This process can be affected by different factors such as stress, changes in weight, or diseases.


  • Reinberg (1967) conducted research in the 1960s which showed that bright light exposure can affect menstrual cycles. Specifically, he found that when women were exposed to bright light in the evening, their menstrual cycles became shorter. This research supports the idea that exogenous zeitgebers, in this case bright light, can affect the menstrual cycle, specifically the luteal phase, by shortening it.
  • One study that supports the idea that exogenous zeitgebers can affect the menstrual cycle is the study conducted by Roenneberg et al. in 2007. The study found that women who were exposed to more natural light (i.e. who had a more “outdoor” lifestyle) had menstrual cycles that were more closely aligned with the natural lunar cycle, compared to women who were exposed to less natural light. This suggests that exposure to natural light can act as an exogenous zeitgeber and help regulate the menstrual cycle.
  • Evolutionary psychologists claim that the synchrony of menstrual cycles in groups of women provides an evolutionary advantage because it allows for better coordination and cooperation among women in a group. This can be advantageous for a number of reasons, such as being able to better care for children and hunt for food. Additionally, synchrony of menstrual cycles may also increase social bonding among women. This may be beneficial in terms of forming alliances and support networks, which can be important for survival in prehistoric times.
  • Penton-Volk et al. (1999) conducted research on the relationship between menstrual cycle phase and social behavior in women. They found that women in the luteal phase of their menstrual cycle (the phase after ovulation) were more likely to engage in prosocial behaviors, such as helping others and cooperating with others, compared to women in the follicular phase (the phase leading up to ovulation). This suggests that infradian rhythms, such as the menstrual cycle, can also play a role in regulating behavior.
  • It is hard to separate infradian rhythms from all external zeitgebers therefore research may not be valid when attempting to isolate the infradian rhythm.

Seasonal Affective Disorder

Seasonal Affective Disorder (SAD) is a type of depression that is linked to the change in seasons, typically occurring during the fall and winter months. Exogenous zeitgebers are external cues, such as light exposure, that help regulate our circadian rhythms. Research has shown that decreased exposure to natural light during the fall and winter months can disrupt our circadian rhythms, leading to symptoms of SAD. Treatment for SAD often includes light therapy, which involves exposure to bright artificial light to help regulate the body’s internal clock and alleviate symptoms of depression.


  • Terman’s (1988) study is one of the first studies that supports the role of melatonin in Seasonal Affective Disorder (SAD). Terman and his colleagues used a form of light therapy that involved exposing patients to bright light in the morning, which mimics the natural light-dark cycle. They found that the treatment was effective in reducing symptoms of SAD, such as depression, fatigue, and insomnia. Moreover, Terman and colleagues also found that bright light in the morning, when melatonin levels are low, was more effective in reducing symptoms than bright light in the evening, when melatonin levels are high. This suggests that melatonin, a hormone that regulates the sleep-wake cycle, plays a role in SAD. The research suggests that the cause of SAD is related to a disruption in the circadian rhythm and the effect that it has on the secretion of melatonin, which is responsible for regulating the sleep-wake cycle.
  • Understanding the role of darkness in SAD has led to the development of light therapy to treat SAD. SAD sufferers have reported that daily use is enough to relieve them of lethargy, depression and other symptoms. The therapy works by resetting the circadian rhythm by providing bright light in the morning, which suppresses the secretion of melatonin and thus reducing the symptoms of SAD.
  • Lewy et al. (1980) investigated the relationship between insomnia and seasonal affective disorder (SAD). The study found that participants who had SAD were more likely to suffer from insomnia, suggesting that the lack of sleep may be a contributing factor to the depression symptoms associated with SAD, rather than solely the disruption of bodily rhythms. The study participants with SAD were found to have more difficulties falling asleep, staying asleep, and having a non-restorative sleep, which lead to more symptoms of insomnia. The researchers suggested that the insomnia may be a primary cause of the depression in SAD and that the insomnia should be treated as a core symptom of SAD, rather than simply a side-effect of the disorder.
  • Palinkas et al. (2004) conducted a study that investigated the relationship between the social environment and mood disorders in participants who spent the winter in the Antarctic. The study aimed to examine the role of the social environment in mood disorders, as opposed to the physical environment. The study found that the social environment had a greater effect on mood disorders in the participants, rather than the physical environment.

Light Therapy

Light therapy decreases the level of melatonin by using a light box to increase the amount of light received, the decrease in melatonin will reduce the sleepiness felt by the patient so should be effective in helping reduce the symptoms.

Strong lights (6000-10000 lux equivalent to daylight) are used in the evening and/or morning. The user would have to sit under a light box for 30 to 60 minutes. They could also use a dawn simulator which gradually increases the light in the morning just before waking up.



  • Light therapy has been found to be effective in improving symptoms of SAD, with studies showing that up to 85% of individuals with SAD experience significant improvement in symptoms after using light therapy.
  • Light therapy is non-invasive, safe and easy to use. It can be done at home, and it does not have any serious side effects.
  • Light therapy is convenient as it does not require any preparation or recovery time, and it can be done at any time of the day.
  • Light therapy is often used as an adjunctive treatment with other treatments such as medication or psychotherapy.


  • It is not effective for everyone, some individuals may not respond well to light therapy, and it may not be effective in severe cases of SAD.
  • The treatment must be done consistently and for a specific period of time each day, which can be difficult for some individuals to maintain over the long term.
  • Light therapy may cause side effects such as eye strain, headache, or agitation in some individuals, and it’s not recommended for individuals with certain eye conditions such as retinal disorders.
  • Light boxes can be expensive, so it may not be accessible for all individuals.
  • Light therapy is not a cure for SAD, it’s a way to alleviate symptoms and therefore it’s not recommended to stop treatment abruptly.