Interior or indoor temperature is the range of air temperatures that usually people prefer for the indoor setting which gives the feeling of warmth and comfort. The word optimum refers to the centre point, which is not inclined towards any extreme. So, the simple meaning of optimum interior temperature would be that the atmosphere inside should not be in extremes of hot and cold, rather should strike a balance.
The World Health Organization (WHO) has reached a standard of comfort and warmth for all across the globe which is 18℃ or 64℉. But again, owing to diversity, it is not practically possible to decide one fixed optimum interior temperature for the whole world. It would vary from country to country and also from person to person.
Therefore, a range is chosen instead of a specific value for optimum room temperature by ticking the basic criterions for an ideal interior temperature. The list could include the level of comfort at that temperature, no extremes (not too hot and not too cold), suits in almost all seasons, should suit all kinds of people, should cover the geographic aspect, etc. So the optimum interior temperature ranges from 18℃ to 25℃ or 64℉ to 77℉.
Nowadays there are so many tools available to set the temperature according to what we want it to be. There are air conditioners, heaters, coolers, and so many things. The same house can have different temperatures in different rooms owing to these machines.
If it is hot, AC is switched on and the actual temperature of 25 or 26℃ could be converted to 18℃ within minutes. Or if the temperature is falling down then heaters are there to increase the warmth. So in this era, we have developed technology to optimize interior temperature artificially according to our own needs.
But these artificial methods are harmful to nature and therefore we should find natural temperatures that suit us without artificially bothering the natural process.