Some who follow the RU Navy on hf say there is no distinct and absolute transition from day to night freqs as I've noted for 0600/1800, that stas will use whatever frequencies are propagating.
That stands to reason, however I note a distinct lack of traffic on 8mHz after about 0700 and 12mHz after 1900, so they do qsy near those times as far as I'm concerned.
What must always be kept in mind is what stas the ships are trying to contact, thus if those stas are in daylight or not, and weather there's a need and/or possibility for long haul (higher freq) or nvis (lower freq) hf comms.
Apparently in an effort to enhance signals security, much cw traffic has been moved from long haul to more nvis and localised networks, as well as satcom. Meaning instead of 12464, 8345, and other ranges noted for reliable long distance propagation, they're all down in the 3 and 4mHz range.
This effort to enhance signals security is problematic for us eavesdroppers located far away from the ships and their respective shore stas.
Earlier today (too lazy to get out from under warm covers to see what time it was and convert that to utc or moscow time) on 8mHz, RMUW was calling RIT, RCV, and RIW in turn and not getting any response for all I can tell, as RMUW had traffic to pass but apparently no shore station heard him so no groups were sent on 8mHz. They might have had success on another freq but I didn't copy any.
This series of reality show (ie scripted) vids has some really neat hardware as well as underway operations of interest to anyone who follows RU Navy;
https://www.youtube.com/watch?v=XhCGtc4kWDQ