4AM

Most interesting piece

TED | Talks | Rives: Is 4 a.m. the new midnight? (video)

My first thought was that I am up and about from time to time at 4AM
Men of a "particular age" often get up to go to the bathroom in the night. For me, sometimes it's 3-4AM
If I don't go right back to sleep, I spend sometime getting some work (reading/figuring/writing) done.

Then I flashed back to when we were racing.
The longest was the "24 Hours of Nelson's" (Nelson's Ledges Ohio, near Warren).

Well, the race ran from 4PM Saturday till 4PM Sunday.

4AM was half way
It was also when there was the first hint of dawn. Roughly "Nautical Twilight"
(Wikipedia : At this time, sailors can take reliable star sights of well known stars, using a visible horizon for reference. The end of this period in the evening, or its beginning in the morning, is also the time at which traces of illumination near the sunset or sunrise point of the horizon are very difficult if not impossible to discern (this often being referred to as "first light" before civil dawn and "nightfall" after civil dusk). At the beginning of nautical twilight in the morning (nautical dawn), or at the end of nautical twilight in the evening (nautical dusk), under good atmospheric conditions and in the absence of other illumination, general outlines of ground objects may be distinguishable, but detailed outdoor operations are not possible, and the horizon is indistinct.