It is 3pm. You have answered forty messages, solved two problems that were not yours, and somewhere in there you forgot lunch. Now the screen feels far away, small things feel big, and there is a dull ache behind your eyes.
Most of us call that being tired. Some of us call it a personal failing. A small study from 2012 suggests part of it might be something much simpler.
What the researchers did
Researchers at the University of Connecticut brought 25 healthy young women into the lab on three separate days, eight hours at a time [1]. On two of those days the women exercised to lose water through sweat, and on one of them they also took a diuretic. On the third day they stayed fully hydrated.
On the dehydrated days, the women lost about 1.4% of their body mass in water. Researchers call that mild dehydration.
What they found
Even at that mild level, compared with the hydrated day, the women reported [1]:
- less vigor and more fatigue
- lower concentration
- tasks feeling harder than they should
- more headaches
Here is the part that stuck with us: their scores on most of the thinking tests did not change. The work still got done. It just felt harder, and they felt worse doing it.
What it does not show
This was 25 women in their early twenties, studied in a lab. It does not tell us how much water any one person needs, and it does not show that water fixes every afternoon slump. The study was funded in part by a bottled water company, which is worth knowing.
Why it matters to you
If something as small as a water bottle can change how heavy an afternoon feels, then some of what you have been blaming on yourself may not be yours to carry. It will not fix everything. It is one less reason to be hard on yourself at 3pm.
There is a version of this for the men in your life too. The same lab ran the study in men a year earlier.
Sources
- Armstrong LE, Ganio MS, Casa DJ, et al. Mild dehydration affects mood in healthy young women. J Nutr. 2012;142(2):382-8. Link
