The Body Clock After Death
The moment the heart stops beating, the body begins to cool. This process — algor mortis — follows predictable physical laws that forensic pathologists exploit to estimate the postmortem interval (PMI). Body temperature is the single most reliable early indicator of time since death, usable for approximately the first 24–36 hours before the body equilibrates with its environment.
Newton's Law Meets Biology
Body cooling roughly follows Newton's law of cooling — the rate of heat loss is proportional to the temperature difference between the body and its surroundings. However, biological bodies are not simple objects. The initial postmortem temperature plateau (the body maintains 37°C briefly due to residual metabolic activity) and the insulating effect of subcutaneous fat create a sigmoidal cooling curve that Newton's simple exponential cannot capture.
The Henssge Model
Claus Henssge's double-exponential model addresses these biological complexities. His formula includes two exponential terms — one for the initial plateau and one for the main cooling phase — calibrated against hundreds of real cases. The model accounts for body mass (larger bodies cool slower), clothing insulation, and environmental conditions, producing a nomogram that forensic pathologists use daily worldwide.
Practical Limitations
Temperature-based PMI estimation has inherent uncertainties. Fever at time of death raises the starting temperature. Environmental changes (doors opening, heating turning off) alter the assumed ambient conditions. Water immersion accelerates cooling dramatically. For these reasons, the Henssge method provides confidence intervals, not point estimates, and experienced pathologists always corroborate with other indicators: rigor mortis progression, vitreous potassium levels, and gastric contents.