Evolution Have Enabled Blind Mole

¶ … evolution have enabled blind mole rats to synchronize their SCN activity to light, even though they cannot see well enough to make any use of the light?

The SCN is a way that mammals are able to synch light and visual images from the retina to the brain. However, most of the input to this path is not initiated from simple retinal receptors. This must mean that light and light intensity is not all that is being communicated, since there are a number of animals that are "blind" based on their environment and evolution. For instance, blind mole rats do not have eye muscles or a lens to focus and they have less than 1% of the number of optic nerves in other comparable species. It appears, though, that the reason blind animals can synchronize their SCN activity to light is because they need certain chemicals that are produced in their brains by the presence of light, even though they cannot see. These chemicals are necessary for the survival of the species; some even speculate that it protects the rats from cancer. Even if the rats do not receive direct light input, they can reset their circadian rhythms through changes in light intensity so they know the best time to be underground, to eat, avoid predation and survive. The ability to reset to an appropriate circadian rhythm is also helpful in protecting the species during seasonal changes (e.g. less light in winter, more in summer), and to trigger hormones for reproduction at the optimum time.

2. If you travel across several time zones to the east and want to use melatonin to help reset your circadian rhythm, at what time of day should you take it? What if you travel west?

Melatonin is hormone that is released by the pineal gland that influences the circadian and circandnural rhythms. Typically, the pineal gland secrets melatonin at night to make us feel tired. When we travel, we do not automatically reset our rhythms for a few days, until our body gets used to the new time zone and pattern of light and dark. It appears that melatonin secretion begins about 2-3 hours prior to sleep, thus people who want to use melatonin to reset their sleep/wake cycle should take a tablet about 2 hours before they wish to start their sleep cycle. The hormone resets the biological clock based on receptors in the SCN. If a person wants to fall asleep earlier, they should take a moderate dose of melatonin in the later afternoon. It does not matter what direction one travels, east or west; it is the time that one takes the melatonin to reset the clock. For example, if one was travelling from Seattle to New York City and the flight arrived at 5pm New York Time the person's internal clock would still be registering 2pm. If the person wanted to be asleep around 11pm, then they should take the melatonin at about 8-830pm. If that same person flew from Seattle to Hawaii, and they arrived at 7pm, their internal clock would register 10pm; so they would still want to take melatonin based on the time they want to sleep in the local area.

3. When cats are deprived of REM sleep, longer periods of deprivation -- up to about 25 days -- are associated with greater rebound of REM when they can sleep uninterrupted. However, REM deprivation for more than 25 days produces no additional rebound. Speculate on a possible explanation. (Hint: Consider what happens to PGO waves during REM deprivation.)

REM sleep, or Rapid Eye Movement Sleep, is a normal sleep stage that includes rapid movements of the eyes, lower muscle tone (relaxation) and a low-voltage EEG. In humans in comprises about 25% of sleep, usually closer to morning. These low-voltage wages are called PGO (pons-geniculate-occipital). The loss of REM sleep due to deprivation causes a rebound, usually causing more REM sleep until the loss is accounted for. However, after 25 days there is no additional rebound. This is likely because there is a threshold level at which the body is able to compensate for a lack of sleep. In sleep deprived animals, PGO activity occurs earlier, and in severely deprived animals it might show up during awake time as a hallucination. The lack of additional rebound is probably a fail-safe mechanism that protects the brain chemistry of the animal.