0:34 – Most of life on the planet has evolved along side a pattern of light; for twelve hours organisms have access to light and for another 12 hours they do not. This environment put a pressure on the organisms to develop a mechanism for anticipating when it will be morning and when it will be night. For animals that are active during the day, it would make sense that there would be pressure put on the animals to make their way back to a hiding place when they sense darkness is approaching. Almost every organ in our body has a clock that optimizes its performance based on the time it believes it is.
2:37 – Dr. Panda suspects that when we are born we don’t have the standard twelve-hour light/twelve-hour dark clock that would be typical for a grown person. He thinks that when we are babies, clocks that make up the circadian rhythm are not wired together. Babies also need plenty of food because they are growing very fast; this rewires the clocks further. At about 6-12 weeks, it has been observed that the children will exhibit a twelve hour light twelve hour dark cycle but a bit offset from the cycle of the sun. We have clocks within us but they are not fully connected with each other until about 4-6 months.
4:22 – 10-15% of the protein-encoding genome is regulated by circadian rhythm. 40-50% of those genes deal with metabolism. Forty years ago, when researchers experimented with removing the suprachiasmatic nucleus (SCN), they discovered that the organism will not have a good sense of when to wake up and be active – their clock is randomized. When a different SCN is transplanted into the organism who had its SCN removed, the organism receiving the transplant will get all of its rhythms back.
This makes a good case for a single master clock existing within organisms.
8:46 – Researches wanted to ask: where is the light receiver that sets the master clock? How is it that blind individuals can adjust to different time zones? In patients who needed their eyes removed, it was observed that they had a clock that was not exactly 24 hours, it was a bit less than that. Over the course of 10 to 15 days, their clocks would be inverse of what is typical.
Three groups have discovered melanopsin, a light receptor found in the eye. If someone was blind but still had melanopsin, their clocks would adapt to changes. If melanopsin is gone, subjects would experience an offset in their clocks which would build into a larger offset as the days go by.
There receptors are found in a very small proportion of retinal ganglion cells which are photosensitive. It is believed to be the visual pigment that synchronizes the circadian cycle to the day-night cycle as well as being involved in the control of pupil size and release of melatonin. The peak sensitivity of these receptors is around 480nm.
Once these melanopsin receptors are activated when exposed to small wavelength visible light (blue-ish), a cascade of gene expression mechanisms are activated.
13:29 – Bright light. Bright light is very important for melanopsin. If we were sensitive to dim light (stars and the moon), these sources of light would activate the mechanisms that shouldn’t be active at the time when we see them. We have rhodopsin which helps us see dim lights; to activate the melanopsin receptors, we need powerful light. You may need around 1000 Lux of illumination to activate melanopsin.
14:50 – You need bright light and several minutes of bright light exposure to allow melanopsin to start the functions it is designed to start.
15:19 – After discovering melanopsin, researchers speculated that these receptors are integrated with the central master clock discussed earlier. It turns out that there are many other functions. For example: it connects to a part of the brain that regulates sleep. This is why it is hard to sleep in a well lit room. It is also connected to the part of the brain that produces melatonin, which helps us sleep. When we are exposed to a lot of light, our melatonin production decreases. According to Dr. Panda, we need as much light as possible in the first half of the day and as little light as possible in the second half.
We have a micro-biological explanation for why cloudy days make us depressed; we need that bright light to activate melanopsin.