The study compared the mood of winter swimmers with a control group. At the start of the study there were few differences between the groups, but the swimmers showed several advantages as the research continued.
“Tension, fatigue, memory and mood negative state points in the swimmers significantly decreased with the duration of the swimming period. After four months, the swimmers felt themselves to be more energetic, active and brisk than the controls,” according to International journal of circumpolar health [Int J Circumpolar Health] 2004 May; 63 (2), pp. 140-4.
The study also noted that swimmers who suffered from rheumatism, fibromyalgia, or asthma, reported that winter swimming relieved pain.
Found with the MEDLINE database at Pioneer, Utah's Online Library