The Human Body: 35 Astonishing Human Body Facts – Part 6

There is a reason you don’t remember falling asleep. Falling asleep is a very smooth transition. You make a transition from awake to pre-sleep, light sleep, slow-wave sleep, and then to REM sleep. Memory formation depends heavily on changes. We are most likely to remember something new rather than something that doesn't really alter over time. Falling asleep is a rather slow change so there is limited need to remember it.

Secondly, pre-sleep goes hand in hand with decreased perceptual awareness. With decreased awareness, there is just less content to store. With the decreased perceptual awareness we switch to an internal reflective state, thinking of things rather than learning new things. So you start to think about other things and then there is nothing new to remember.

The white-colored, half-moon-shaped structure at the base of our fingernail is called the ‘Lunula.’ This forms part of the nail matrix, basically the region that’s responsible for the growth of the entire fingernail. The rest of the nail matrix lives under your skin at the base of your fingernail.

Damaging the nail matrix will screw with the growth of the entire nail. You'll sometimes see lines called Beau's Lines if you damage the lunula.

By looking at the distance of these lines from the nail matrix, you can tell approximately how long ago the injury happened. Fingernails grow ~1mm per month and are fully replaced every 6 months. So an injury that has grown halfway up the nail (~3 mm) is about 3 months old.

Someone asked us this question, “Why did historical diseases like the black death stop?” The answer is that people still get bubonic plague in some parts of the world, just not on an endemic scale. Perhaps because of sanitation, it doesn’t spread like it used to, but it has not completely been eradicated.

In general, infectious diseases never keep going forever at a high rate, because as soon as the number of healthy individuals drops significantly (due to infection, immunity, or death), transmission rate drops and so does the supply of fresh host to spread to. This is why deadly diseases only come in sudden waves and die down.

The key to controlling any infectious disease is to reduce the number of susceptible people by any means possible (vaccine, quarantine, or getting them all infected). It’s called the SIR model (Susceptible-Infected-Removed). It’s the basis for all modern infectious disease models.

Everyone knows about the Second Plague Pandemic (which includes the Black Death) but there were three major pandemics in total. The First Pandemic spread in the 6th and 7th centuries which killed up to 40% of the population in Constantinople and Europe. It happened during a period of scarce historical record so it's now mostly forgotten but it devastated the Byzantine Empire.

The Second Pandemic began with an epidemic in Mongolia in the 1330s and then it spread to Europe through the Silk Road. It was first recorded in Europe in 1347, resulting in a 6-year period called the Black Death where an estimated 30% to 60% of Europeans died. The end of the Black Death didn't result in the end of the Second Pandemic as the bacteria became endemic in Europe and continued to cause deadly Bubonic plague epidemics for centuries to come. During the 16th to 17th century, there was a major plague outbreak in Paris, on average once every 3 years. The classic plague doctor outfit wasn’t invented until 1619 and used until 1656. The last major British plague epidemic was the Great Plague of London from 1665-1666 which also spread to the surrounding areas. This resulted in Newton being sent home from the University of Cambridge and quarantined. 1666 was Newton’s annual mirabilis when a bored 23-year-old Newton came up with numerous theories and experiments which changed the history of science.

The Third Plague Pandemic lasted from 1855 until 1960 in India and China but didn't really spread to Europe. It caused the death of 12 million people, 10 million in India alone.

Some researchers also believe that the Bubonic Plague might also have been responsible for the Neolithic decline of the human population in Europe 5000 years ago.

People often wonder why grief is physically exhausting. We often miss the fact that emotions aren’t just feelings, they’re biochemical reactions. Grief is induced by and induces a lot of stress chemicals (like cortisol) and you don’t get enough of the happy chemicals and endorphins. Your body doesn’t function well in this state.

Then there's also the fact that your brain is very energy-hungry, so any time you use it a lot, you will feel tired, e.g. studying, or jobs that require frequent decision-making. The simple act of thinking about the person you miss all the time uses up a lot of energy. On top of that, people who are grieving often don't replenish the energy used because they are sleeping and eating less.