Worn Out: Women look their oldest every Wednesday at 3:30 p.m.
The research shows one in ten women (12 percent) find Wednesday the most stressful day in a typical week.
But they reveal Thursday is the day they are most likely to have sex.
It gives them a youthful rosy glow which could contribute to why women feel so happy on a Friday - 60 percent name it as their happiest day.
A quarter of women feel stressed several times a week - while one in five (19 percent) admit they will work through Wednesday's lunch hour.
The study, carried out by the tanning brand, St Tropez, revealed two thirds experience a "slump in energy levels" mid-afternoon every Wednesday.
This forces a quarter of women to reach for a sugary snack on a bid to boost their mood and energy.
Nichola Joss, St. Tropez skin expert, said: "It's fascinating that 3.30pm on a Wednesday is the time women look their oldest.
"Combine the highest stress levels of the week on a Wednesday with the natural mid-afternoon slump and it seems that's why women can look older than their years."
And Ms Joss revealed how a woman's sex life can have a big impact on how they feel.
She added: "Many women credit a healthy sex life for keeping them looking and feeling young.
"As the mid-week slump passes and stress levels begin to decrease, it's fascinating that Thursday is the evening they are most likely to feel amorous."
Almost half of women (46 percent) are likely to indulge in a boozy treat at the weekend - another reason why women look their eldest mid-week.