What is rigor mortis, and why does it happen?

After death, a series of natural processes begin within the human body. The corpse cools, pales and stiffens before it begins to decompose. The stiffening of the corpse is called rigor mortis. It's viewed as macabre, creepy and unsettling, and it's been used as a plot device in crime shows, a jump scare in horror films, and a clue in movie mysteries.

But what, exactly, is this process, and why does it happen to nearly all bodies?

Rigor mortis is a natural process that occurs at a cellular level. It begins immediately after death as the body runs out of adenosine triphosphate, the energy molecule commonly known as ATP. "You can essentially think of [ATP] as your fundamental source of energy for muscle contraction," said Dr. Michelle Jorden, chief medical examiner of Santa Clara County and president of the National Association of Medical Examiners. "The ATP molecule is what's needed for us to move."

After death, the body stops producing ATP, and its supply of the energy molecule is depleted within a couple of hours, on average, Jorden said.

The body needs ATP not only to move but also to relax. As the body's supply of ATP runs out, the muscle filaments — proteins bound together within muscle cells — begin to stick together.

One misconception is that the body snaps into rigor instantly. While rigor mortis starts immediately in all muscles following death, in most cases it can take several hours before it becomes visible to the naked eye as filaments stick together.

Between two and six hours after death, stiffness first appears in the body's smallest muscles in the face. After six to 12 hours, it progresses through larger muscles in the hands, arms and neck and chest. Finally, the body's largest muscles in the lower body stiffen — full-body rigor mortis happens between 12 and 24 hours after death, completing this sequence known as the "march of rigor."

Another misconception about rigor mortis is that it's permanent — that, once stiffened, the body will stay that way forever. "It's a transitional phase, not a permanent state," said Shawn'te Harvell, a licensed funeral director in New York and New Jersey and president of the National Funeral Directors & Morticians Association.

Rigor mortis lasts around 24 to 48 hours and then disappears once decomposition begins, when "muscles become flaccid again as the decomposition breaks down protein structures," he said. The body becomes flaccid in the same order it stiffened — starting with the face and hands, down the neck and torso, and then the legs.

This timeline is a guideline, Harvell said, and the process is unique for each body. Factors such as temperature, age and activity level at the time of death; overall health and body composition; and medications taken during life affect how quickly or slowly the body goes through rigor mortis before decomposition sets in. "No two cases follow exactly the same timeline or intense pattern," he said.

Jorden, a practicing forensic pathologist and neuropathologist, said that determining the timing of rigor mortis can be "a very valuable tool" for assessing the scene of a death.

"[It] can actually provide clues if the body has been manipulated or moved," she said. What's more, if the timing of rigor mortis doesn't quite line up with what they believe the circumstances of death were, it may be a clue that "causes us to pause and ask more questions."