Pick a sleeping bag for the temperature.
A sleeping bag's rating describes the bag in a laboratory, not your night on the ground. Three corrections separate the two. Individual metabolism is the largest — a cold sleeper effectively loses about 4 C of rating and a warm sleeper gains 3. The sleeping pad is the one people most often get wrong: insulation beneath you is compressed to almost nothing by your body weight, so conduction into the ground becomes the dominant heat loss path. Below about R 3 a pad actively costs you warmth the bag cannot make up. A thermal liner adds a genuine few degrees for little weight and is the cheapest way to extend a bag's range.
Effective rating
Effective C = bag rating + sleeper adjustment + pad penalty + liner bonus
Pad penalty
Penalty C = (4 - R-value) x 2.5, zero at R 4 or above
A general guide, not a safety rating. Sleeping bag performance depends on fill condition, moisture, wind, shelter, clothing, food intake and individual physiology, and manufacturer ratings vary in how they are tested. Hypothermia is a serious risk in cold conditions — always carry a margin of warmth beyond the forecast and seek qualified instruction for winter or alpine camping.
Because the down or synthetic fill underneath you is crushed flat by your body weight and provides almost no insulation. Heat then conducts straight into the ground, which is a far better heat sink than air. Below R 3 the pad, not the bag, is what determines whether you are cold.
The EN and ISO comfort rating is the temperature at which a standard cold sleeper sleeps comfortably; the limit rating is where a standard warm sleeper is curled up but not shivering. Use the comfort rating for planning — the limit rating describes survival, not sleep.