Eucalyptus, Rosemary, Sage Dying

I was at Richters in the spring, most of the plants and herbs I bought are doing great but recently things have been going downhill. My eucalyptus is really struggling, there is still some new growth but all the leaves have turned brown and fallen off. Any ideas? The sage just up and died and now the rosemary, which had been doing beautifully, is doing the same. Nothing has changed with how they have been cared for. They are all indoors and in the sunniest window I have. Please advise?? Should they be placed in a darker corner for a dormant period? Could the colder outdoor temperatures be causing this, the herbs are on the window sill. On the up side the tea tree and Greek oregano are growing like there is no tomorrow.

You probably are growing on the dry side and with cooler weather the humidity on the windowsill probably went down as the heating system came on intermittently. This would have dried the plants out faster and if the rootball dries out completely for some plants, they cannot recover. Eucalyptus and rosemary (and myrtle too for that matter) are known for their intolerance of dry roots. It takes a fair length of dry time to kill sage, but perhaps the plant dried out and you missed it at the the regular watering. Tea tree also does not like to ever dry out, but perhaps it was in a larger pot and therefore does not dry out so quickly. Oregano is quite tolerant of dry conditions and was not bothered by the dry spell.
At this time of year, in autumn, the amount of light the plants are getting is falling as the days become shorter and the intensity of light declines due to the changing angle of the sun. Some plants may be experiencing a natural slowdown in growth as the light diminshes, and the onset of that effect can vary with the different herbs. If that is what is happening, you need to exercise greater care in watering not to overwater during the slower growth winter period, or you need to supply more light in the form of supplemental artificial light. If we are adding light, we like to set up the light fixture right in the window on a timer so that plants receive natural light during the day and artificial light from early morning to early evening.