Fire May Have Transformed Humans From Daytime Creatures Into 24-Hour Animals… Kind Of Like Lemurs

• A study suggests humans have a similar day-to-night time activity distribution to lemurs, with far more nighttime activity than other primates like gibbons.
• Dr David Samson at the University of Toronto, Canada, thinks we weren't always like this, and became capable of "extending activity into the night when the social and ecological payoffs were high enough.”
• Samson hypothesizes that the invention of fire was central to this, as it made the night "usable" and facilitated greater social bonding after dark.

Humans are generally considered to be diurnal creatures, which means we’re naturally active during the day and asleep at night. However, a new paper challenges this idea, suggesting that humans – like lemurs – are actually cathemeral, meaning our business hours aren't constrained by the rising and setting of the Sun.

“Our findings suggest that humans evolved a remarkable capacity for activity across the 24-hour cycle,” said study author Dr David Samson at the University of Toronto, Canada. 

“We appear to be more cathemeral than traditionally assumed, though not necessarily in exactly the same way as classic cathemeral primates such as some lemur species,” he told IFLScience.

Analyzing data from the Indigenous Hadza hunter-gatherers of Tanzania, Samson found that individuals were active for an average of 52.31 minutes per hour during the day and 24.10 minutes per hour at night. 

This gives a day-to-night time ratio (DNTR) of 2.36 – a figure that is remarkably similar to the DNTR of lemurs, which was calculated to be 2.42.

Gibbons, meanwhile, are strongly diurnal and were found to have a DNTR of 35.60. Such a finding suggests our activity pattern is considerably more aligned with cathemeral primates than those with a strict bedtime. 

“Most primates are strongly tied to either day or night, but humans are different,” said Samson. “We are basically diurnal animals, but we became capable of extending activity into the night when the social and ecological payoffs were high enough,” he adds.

In other words, our circadian system - which sets the tone for our daily activity cycle – remains anchored to the solar day, yet our behavioral adaptations have enabled us to develop a highly flexible, cathemeral schedule. 

To explain this, Samson proposes what he calls the social sleep theory, which suggests that humans have created a unique “sociotechnological niche” that affords us the ability to redistribute our activity throughout the day and night.

“Fire was central,” he said. “It changed the sensory landscape after sunset, reduced predation risk, extended social time, enabled cooking and warmth, and created a protected space for conversation, ritual, planning, teaching, and vigilance.” 

“In other words, the night became usable,” said Samson. In ancestral settings, this may have provided a number of advantages by allowing people to divide their work schedule into tasks that were best performed during the day and those that could be achieved while sitting around a campfire. 

It also might have allowed for greater social bonding after dark and enabled hunter-gatherers to adapt to diverse environments.

However, in modern environments, overly-cathemeral behavior patterns can clash with our natural circadian rhythm and exceed the limits of our biological capacity. For instance, Samson describes how “chronic artificial light at night, shift work, social jet lag or constant sleep restriction” have all become commonplace in the modern world, yet may push us towards a level of cathemeralism that is detrimental to our well-being.

So while fire may have helped prehistoric humans break free from the day-night cycle, that doesn’t mean we should stay up all night.

The study has been published in the Journal of Human Evolution.