Witching hour

In folklore, the witching hour or devil's hour is a time of night, typically between midnight and 1 am, that is associated with supernatural events, whereby witches, demons and ghosts are thought to appear and be at their most powerful. Black magic is thought to be most effective at this time due to the veil between worlds being thinner. The term now has a widespread colloquial and idiomatic usage that is associated with human physiology and behaviour to more superstitious phenomenon such as luck.
Origins.
The phrase "witching hour" began at least as early as 1775, in the poem "Night, an Ode." by Rev. Matthew West, though its origins may go further back to 1535 where the Catholic Church prohibited activities during the 3-4 am timeframe due to emerging fears about witchcraft in Europe.
In the Western Christian tradition, the hour between 3 and 4 a.m. was considered a period of peak supernatural activity – this time is also referred to as the "Devil's hour" due to it being a mocking inversion of the time in which Jesus supposedly died, which was at 3 pm.
Time.
There are multiple times that can be considered the witching hour. Some claim the time is between 12 and 1 a.m., while others claim there is increased supernatural activity between sunset and sunrise. The New Zealand Oxford Dictionary identifies midnight as the time when witches are supposedly active. 
However, during the time in which this term originated, many people had sleeping schedules that meant they were awake during the middle of the night. Nonetheless, there is psychological literature suggesting that apparitional experiences and sensed presences are most common between the hours of 2 and 4 a.m., corresponding with a 3 a.m. peak in the amount of melatonin in the body.
Physiology.
The witching hour may stem to a human's sleep cycle and circadian rhythm – the body is going through REM sleep at that time, which is the deepest sleep, where the heart rate is slower, body temperature reduced, breathing pattern and blood pressure irregular. Therefore, sudden awakening from the deepest sleep would cause agitation, fear and disorientation in an individual. 
Also, during REM sleep, which usually occurs within the witching hour, unpleasant and frightful sleep disturbances such as parasomnias can be experienced, which include nightmares, rapid eye movement sleep behavior disorder, night terrors, sleepwalking, homicidal sleepwalking and sleep paralysis.
Moreover, during the night and well into the witching hour, symptoms of illnesses and conditions such as lung disease, asthma, flu and common cold seem to exacerbate because there is less cortisol in the blood late at night and especially during sleep. As such, the immune system becomes very active and white blood cells fight infections in the body during sleep, and this would thereby worsen the symptoms of fever, nasal congestion, cough, chills and sweating.
Colloquial usage.
The term may be used colloquially to refer to any period of bad luck, or in which something bad is seen as having a greater likelihood of occurring. 
In investing, it is the last hour of stock trading between 3 pm (when the U.S. bond market closes) and 4 pm EST (when the U.S. stock market closes), a period of above-average volatility. 
The term can also refer to a phenomenon where infants or young children who cry for an extended period of time during the hour (or two) before their bedtime, where they'd usually be irritable and unwieldy with no known cause.
To reduce gun violence, curfew hours in Washington D.C. have been in force between 11 pm and 12 am to lower juvenile gunfire incidents. Influenced by the idea of "witching hour", 11:00–11:59 pm on weekdays are referred to as the “switching hour.” Furthermore, violent crimes like rape and sexual assault would peak at midnight on average and DUI police incidents would usually tend to occur at around 2 am.