'In a dark place'
Yesterday, after feeding my starters (thanks again, Benny!), I left them away from (direct) sunlight for most of the day.
But with nothing actually covering or obscuring my sealed Weck jars.
In the late afternoon - probably about four hours later - I covered both jars with an opaque cloth.
Both starters then doubled in size.
This makes me wonder whether starters do better without any kind of light!
Or was it just that by the end of the day (during which room temperature had been almost constant, maybe even a little cooler later) more time had passed?