Back in the days when I frequently went sinking, I did so under the darkness of morning. I actually prefer to see what I'm doing as it's part of the fun, but in certain sinking spots, others can see you by day.
I recall one early Sunday morning wanting to check out a fairly large pond near a drive-in theater. I'd parked the car away from the site, made my way down to the pond, and went wading. It was rather disappointing since it was all water and very firm mud. No matter where I went, there was no spot to sink in. Then above me up the slope of the land towards the parking lot of the drive-in theater I saw the near parallel beams of headlights sweeping across the dark sky. Then came the front of a car coming to a stop far above me and the silhouette of emergency lights against the star-lit background... a city police cruiser!

I quickly retraced my track, wadding along the edge of the pond towards where I parked the car. Fortunately, nothing happened in this incident. Would've been hard to explain to a local police officer why I was wading in a pond early in the morning.
My all-time favorite sinking spot was a playa lake near a soda distribution center on the outer edge of the city. During the summer the water level dropped considerably to expose some really fine mud. The depression of the playa lake was deep enough that no one could see me from the highway even by day; however, anyone would have a clear view of me from the parking lot of the soda distribution center, and there was often evidence left behind that people did frequent the area during the day. This place had the best, the thickest, and the deepest mud, and was most enjoyable to sink in. So, I'd bike out to it early under the darkness of a Sunday morning, reach the area as dawn just lit the sky, and would enjoy an hour or more of wading and sinking as the morning light intensified. When I could tell by the sound of increased traffic on the highway above me, I'd wrap it up and bike home.
So, yes, I can vouch for sinking under darkness, though I shot for early morning so as to provide some morning light to see what I was doing.