The visible saving is only one layer
An overnight bus or train can look efficient because it combines movement and lodging into one line item.
For some itineraries, that works well. But the visible saving may hide lower sleep quality, a weaker next day, or extra spending caused by arriving too tired to make good decisions.
Why solo travelers feel the tradeoff more sharply
When traveling alone, there is no one else to help absorb the cost of fatigue. You make the navigation decisions, check-in decisions, transport decisions, and safety judgments yourself.
A better frame
Instead of asking whether overnight travel is cheaper, ask whether it leaves you in a usable state for the next stage of the trip.
That is often the more important economy.