Effect of Far Infrared Reflecting Sleepwear on Recovery and Sleep

Far infrared light (FIR, 5.6-1000 µm) penetrates our skin and reaches the underlying tissue up to 4 cm and promotes widening of the blood vessels (vasodilation). This results in increased blood flow in the tissue under the skin, without heating up the skin itself. Increased blood flow allows more oxygen to reach the muscles and more metabolic waste products to be transported away from the muscles. Thus, exposure to FIR might aid the process of recovery in muscle groups lying close to the skin.

Indeed several research groups could show that exposure to FIR aids recovery. 30 min intense far infrared exposure enhanced recovery from exercise-induced muscle damage in highly trained runners. Studies assessing clothing containing an FDA approved ceramic particles covered yarn that reflect FIR light showed delayed onset muscle pain decreased and a reduction in inflammatory markers.

Since a large part of our recovery takes place during sleep it is of interest to assess the effect of FIR on night-time recovery and sleep itself. One study showed that FIR exposure during sleep increased the amount of non-rem sleep in rats. This effect has not been replicated in humans, possibly because increased temperature is not beneficial for sleep and thus should be kept constant. The aim of this study is to investigate the effect of far infrared reflecting sleepwear on night-time recovery and sleep.

Sleepwear of a breathing and thermoregulating material, with a layer of FIR reflecting minerals printed on the outer surface, will be assessed. Physically active males are invited to participate in two bouts of exercise in the lab, separated by one week. The exercise bout targets one leg, the second week the other leg is exercised. Starting the protocol with the dominant/non-dominant leg (determined in first session) will be randomized. Participants will then wear the FIR reflecting sleepwear and placebo sleepwear respectively at home for the 7 nights following each bout of exercise.

To assess sleep quality activity will be measured using wrist worn activity trackers. To assess recovery physical performance will be measured (max. single leg force and jump height), knee range of motion and thigh circumference of each leg at before each exercise bout, after each exercise bout and at three days after each exercise bout. Per exercise bout muscle soreness will be induced in one leg of the two legs respectively, to account for a repeated bout effect.

It is hypothesized that more overnight recovery takes place during nights with FIR reflecting sleepwear compared to nights with placebo sleepwear.

Inclusion criteria

• Physically active (>3h of sports per week)
• BMI < 25

Exclusion criteria

• Sleep problems (assessed using standardized questionnaire included in entrance questionnaire)
• Cardiovascular problems (self-reported in entrance questionnaire)
• Sensitive skin/allergies that present on a skin level (self-reported in entrance questionnaire)
• Knee pain while exercising (self-reported in entrance questionnaire)
• Night shift work during study period