The skin condition acne may be worsened by stress, research has found.
Acne is a common skin condition
Scientists found that college students with acne were vulnerable to a worsening of their condition during examination periods.
Acne vulgaris is the most common skin disease treated by dermatologists - affecting an estimated 85% of the population at some time in their lives.
However, although the condition is so widespread, there is still considerable uncertainty about which factors cause, or exacerbate it.
Many people believe that stress does play a role in acne flares - but there has been little research to support this claim.
The researchers, led by Dr Alexa Kimball from Stanford University, studied 22 university students with varying degrees of acne.
The students' acne was rated for severity during a non-exam period (approximately one month before an examination) and again during an exam period (three days before an exam to seven days after an exam).
The students also filled out questionnaires during the two acne assessments, in which they rated their stress levels.
The researchers found the students had worse acne during exam periods, when they also rated their stress as higher.
The link remained even after other factors such as changes in sleep hours, sleep quality, diet, and number of meals per day were taken into account.
Writing in the journal Archives of Dermatology, the researchers say the finding does not prove that stress directly causes acne.
"It is certainly plausible that the correlation observed is in part due to worsened acne itself causing increased stress, instead of the reverse relationship.
"However, in a high-achieving population such as university students, subjects tended to report becoming less concerned with their appearance during examinations.
"Thus, it is more likely that increasing stress exacerbates acne instead of the reverse relationship."
Various mechanisms have been proposed for why stress may potentially aggravate acne.
Some investigators believe that stress stimulates the release of hormones known to worsen acne by increasing production of oily substances from sebaceous glands found in the skin.
There is also research suggesting that stress increases production of chemicals which can trigger an inflammation.
In addition, stress is known to slow down the wound healing process by up to 40%.
Dr Niall Wilson, a dermatologist at the University of Liverpool, told BBC News Online he was not surprised by the findings.
"Lots of patients describe stress as a factor which flares their skin.
"However, it is by no means the only factor - lots of things interact.
"It may also be that if somebody is depressed or anxious then that can have a significant impact on how they view the condition of their skin."
Dr Wilson said little research had been carried out into the factors influencing acne. For instance, it had long been assumed that diet played no role in the condition, but this had only recently been challenged.
"It is just not a sexy subject for research because it is not a life-threatening condition, but it does cause serious problems for people who get it."
Research has also found that stress may exacerbate the symptoms of other skin diseases, such as psoriasis, alopecia areata and atopic dermatitis.