www.casa.gov.au/avreg/fsa/download/99nov/FSA22-29.pdf
In the US in 1997, more than 80 per cent of fatal GA weather-related accidents were caused by visual flight into IMC (Instrument Meteorological Conditions)....
"In some cases, flying at night without sufficient references can be more hazardous than inadvertently flying into cloud,"
...
FLYING AT NIGHT IS IN MANY respects more challenging than day VFR flight, for a variety of reasons. Chief among these is the lack of good-quality outside visual cues and the consequent increased risk of visual illusions and spatial disorientation. There are many more hazards associated with night flight apart from the graveyard spiral, and JFK Jr?s accident serves as a timely opportunity for reviewing these. As a pilot, the more you know and understand about these hazards, the safer you are likely to be at night.
...
Human beings are not designed for aerial operations, either by day or by night.We can only achieve this by the use of technology and training. Our physiological limitations become very evident under certain flight conditions, and the night environment is perhaps the most significant.