Biologists from the UK have found evidence that our circadian rhythms are causing us to feel stiffer in the morning than we do at any other time of day.
This means that our stiffness isn’t simply from laying still for hours, but a biological response that’s dictated by how our bodies repress inflammation during the night.
To investigate the phenomenon of morning stiffness, researchers from the University of Manchester induced arthritis in mice by injecting their joints with collagen. They then observed how the arthritic joints inflamed over the course of the day, finding that different hours of the day produced different results.
During the night cycle – which the researchers controlled with a series of lights – the mice experienced very little inflammation, and inflammatory markers were reduced. But after the lights were turned back to simulate daytime, the paws of the arthritic mice were significantly more swollen.
The team says these differences in inflammation stem from specific cells inside mouse joints called fibroblast-like synoviocytes (FLS), which run on a 24-hour cycle, similar to our own body clocks.
These dictate the amount of inflammation, and therefore pain, the mice experienced in their joints by producing certain proteins.
To confirm their findings in something that’s not affected by arthritis, the team worked with cultured mouse FLS cells, removing the ‘clock proteins’ that regulate the day/night cycle to see what would happen.
As it turned out, the cells stayed inflamed throughout the day without this clock mechanism.
The researchers then injected the mouse cells with a drug that triggered the same anti-inflammatory proteins that the FLS cells typically do, which ended up reducing inflammation markers. They did the same with cultured human FLS cells, and saw the same results.
These findings suggest that inflammation is caused by a series of complex biological systems that work together to create a daily cycle.
At night, inflammation drops – though the team doesn’t speculate on why – and slowly ramps up during the day. It will take further study to understand why these systems are in place.