Okay as a personal example I like to take warm showers - they’re extremely comfortable and relaxing - essentially a makeshift sauna. I find that if I’m not deliberate enough about actually completing the shower, I can get lost in my own thoughts and spend over 30 minutes showering. Now I’m certainly more introverted than average, but I would guess that a lot of other people can likewise get lost in their own thoughts in a sufficiently relaxing environment.
There is some stimulation there, and you have a task to accomplish even if you don't go about it very steadily. I'm comfortable going for long walks outside alone with my thoughts, even if its far too cold. But the cold, or the sunlight, or the breeze is all stimulation. Being alone in a room for a period with no task to accomplish - or to not accomplish - without stimuli is definitely less comfortable.
