Endocrine Signalling & Reproductive Biology 2

Photoperiod: Refers to the environmental light/dark cycle. Humans are diurnal creatures, meaning we are active during the day and sleep at night.
- Melatonin levels are high at night and low during the day, following the photoperiod changes.
- Seasonal changes impact the duration of light and darkness, especially noticeable in places like Aberdeen during winter.
- Animals show varying melatonin profiles based on their activity patterns and environmental cues.

Ultradian Cycle: Approximately 2 days in duration; also referred to as a circadian cycle (around 24 hours).
Circannual Cycle: Lasts about a year and is often seen in seasonal breeders.

Humans evolved from diurnal species, reflecting a deep biological lineage.
Key role of melatonin: synchronizes physiological processes with the external environment, which is crucial for daily and seasonal biological functions.

Besides rods and cones (which detect light and color), humans have melanopsin-containing cells (retinal ganglion cells).
- These cells act as light sensors and are involved in regulating the circadian rhythm.
- Some blind individuals report light sensory perception likely due to these cells.
The signaling pathway involves the retina-hypothalamic tract, leading to interactions with the suprachiasmatic nucleus and the pineal gland, which regulates melatonin secretion.

Hormones like melatonin, TSH (thyroid-stimulating hormone), testosterone, and insulin have specific daily peaks, which illustrates the importance of circadian cycles.
The relationship between hormone peaks and circadian patterns can be studied through:
- Shift Workers: Exposure to irregular working hours leads to various health issues, including increased cardiovascular disease risk and reduced lifespan.
- An experiment with continuous wakefulness shows how melatonin increases during sleep-wake cycles but varies in response to artificial light exposure.

Factors like artificial lighting and changes in the environment (e.g., food availability, weather) influence hormonal cycles in both humans and animals.
Historical data from Rotterdam (1873-1887) maintained by the Dutch indicates seasonal events in human reproduction, with notable fluctuations in pregnancy probability across the year.

Sheep: Exhibit strong photoperiodic responses; they become reproductively active based on changes in day length, with distinct phases of melatonin elevation influenced by seasonal changes.
- The longer nights in winter correspond with longer periods of high melatonin.
Humans: Have a phase-shifted melatonin response in relation to seasonal changes:
- In winter, melatonin rises similarly but decreases more slowly compared to summer.
- There is also a rapid drop in melatonin levels at dawn in summer, suggesting a more complex relationship with the photoperiod.
Similarities in Elephants: Some animals, like elephants, also show phase delays in melatonin responses akin to humans, contrasting with the clear photoperiodic responses seen in sheep.

Melatonin and photoperiod are crucial in regulating biological functions and reproductive cycles in both humans and animals, although mechanisms may vary. The relationship between light exposure, hormonal patterns, and overall health underscores the intricate connections between our biology and environmental changes.