Tog is a measure of warmth, and sleep bags are usually rated 0.5, 1.0, 2.5, or 3.5 tog. As a general guide: 0.5 tog suits hot summer nights, 1.0 tog suits mild spring/autumn rooms, 2.5 tog covers most of the year in a typical UK home, and 3.5 tog is for a cold room in winter with no heating overnight.
The room thermometer matters more than the calendar date — a consistently warm nursery in December might still call for a 1.0 tog bag, while a chilly room in April might need 2.5. Aim for a room temperature around 16–20°C rather than dressing by season alone.
A simple check for overheating: feel the back of baby’s neck or chest (not hands or feet, which are naturally cooler). Sweaty or hot to the touch means too warm; cool to the touch is generally fine.