Far Infrared Therapy for Sleep

Introduction

Far-infrared radiation has been regarded to be indifferent to living organisms, since the limits of radiation effective in inducing a physiological reaction are considered to lie between 300 and 950 nm (Wolken 1971). Recently, we reported that rats continuously exposed to far-infrared radiation from the prenatal period exhibit an acceleration of body growth (Inou~ and Honda 1986). This paper describes the first attempts to detect a sleepmodifying effect of the far-infrared radiation.

Materials and methods

Male rats of the Sprague-Dawley strain were raised in our closed colony on a 12 h light and 12 h dark schedule (lights on: 08.00 h to 20.00 h) under a constant air-conditioned environment of 25 _+ 1 ~ C and 60 _+ 6 % relative humidity. They were allowed free access to rat chow and water. At the age of 6~70 days, animals weighing 300-350 g were implanted with three cortical electroencephalogram (EEG) and two nuchal electromyogram (EMG) electrodes. The surgical procedures are described elsewhere (Honda and Inou~ 1978). The rats were individually isolated to a special recording cage, in which two ceramic disks (diameter: 40mm, thickness: 5 mm), coated with a thin layer of either far-infrared irradiative film or non-active control film, were placed on the floor. The experimental disk was proven to radiate a band of 4-16 gm far-infrared rays (energy intensity: 12.(~71.5 kcat/m 2 per hour at 60 ~ C). A slip ring connecting the lead wires and a polygraph allowed free movements of the rats. After a week's acclimatization in the recording cage, the EEG, EMG and locomotor activity of each rat, exposed either to the experimental or the control disks, were continuously monitored for 9-10 consecutive days and processed by routine computer-aided methodology (Honda and Inou6 1978, 1981)~ Amounts of slow wave sleep (SWS), paradoxical sleep (PS) and wakefulness (W), and the number and duration of their episodes were calculated separately for the light and dark periods. Statistical analysis between the experimental and control groups was done by the Student's t-test.

Results

The results are summarized in Figs. 1, 2 and Table 1. In the light period, the hourly amount of
SWS in the experimental group (n = 16) slightly but significantly (P < 0.01 at I 0.00 h) exceeded that of the control group (n = 16) during the early phase (Fig. 1, upper traces). The total amounts of W and SWS significantly decreased and increased, respectively, during the first trisection (Table 1). Cumulative values of SWS between 11.00 h and 17.00 h exhibited a significant elevation from the control level, although the total amount in the 12 h period was not significant (Fig. 1, below). The increase in SWS during the light period was largely due to a prolongation of episode duration as compared to the control group (Fig. 2, upper traces; Table 1). The number of SWS episodes decreased. On the contrary, no difference was found in PS parameters between the two groups. In the dark period, SWS parameters for the experimental group exhibited time-course fiuctua- tions as compared to the control group. The first
trisection was characterized by trends similar to those occurring in the light period (Fig. 2, upper traces). A reciprocal crossover occurred in the second trisection and the values for both the duration and frequency of SWS reached control levels. The hourly amounts of both SWS and PS showed a significantly decreasing trend towards the end of the dark period in the experimental group (Fig. 1, upper traces). However, cumulative values were almost identical between the two groups (Fig. 1, betow). A significant difference was found between the two groups in the total duration of PS and W for the third trisection (Table 1). This was largely due to a prolongation of episode duration of W and a reduction of that of PS (Fig. 2, below; Table 1).

Fig. 1. Time-course changes in slow-wave sleep (SWS) and paradoxical sleep (PS). Upper: hourly amount; below: cumulative amount in the light (L) and dark (D) periods. The thick and thin lines represent the average values from far-infrared irradiated rats (n = 16) and those from control rats (n = 16), respectively. Single and double asterisks indicate that the difference between the experimental and the control groups is significant at P<0.05 and P<0.01, respectively

Fig. 2. Percentage changes in frequency (F) and duration (Du) of episodes of slow-wave sleep (SWS, upper) and paradoxical sleep (PS, below) of far-infrared irradiated rats as compared to those of the control rats averaged at 4 h recording intervals in the light (L) and dark (D) periods. Single and double asterisks indicate that the difference between the experimental group and the control group (baseline) is significant at P<0.05 and P< 0.01, respectively

Discussion

Rats are night-active and day-resting animals: approximately two-thirds of their sleeping time appeared in the light period in our young male Sprague-Dawley rats (Honda and Inou6 1981). Such a circadian rest-activity rhythmicity was little affected by the continuous exposure to the farinfrared radiation. However, the treatment exerted an SWS-promoting influence in the early light period and an SWS/PS-suppressing effect in the late dark period. Thus, the treated animals demonstrated an enhanced contrast of more rest in their resting phase and more activity in their active phase, although the daily amounts of SWS and PS remained unchanged. At present, no information is available as to the mechanism involved in this phenomenon. It
seems likely that the folklore concerning the health-improving properties to man of far-infrared rays (Yamazaki 1987) might possibly be related to an improved sleep state. In this connection, the acceleration of growth in rats exposed to far-infrared radiation (Inou6 and Honda 1986) may be explained by kind of sleep improvement, which causes an increased secretion of growth hormone. Further investigations will clarify such a causality

Table 1. Total amount (7) in minutes, frequency (F) in number per 4 h and duration (D) in minutes of episodes of wakefulness
(IV), slow wave sleep (SWS) and paradoxical sleep (PS) in far-infrared irradiated and control rats (mean • SEM)

* P<0.05; ** P<0.01
Acknowledgements

The disks used in this research were kindly supplied by Nishikawa Sangyo Co. Ltd., Tokyo. This investigation was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan (No. 60126002) and the Naito Foundation, Tokyo

References

Honda K, Inou~ S (1978) Establishment of a bioassay method
for the sleep-promoting substance. Rep Inst Med Dent Eng
12:81-85
Honda K, Inou6 S (1981) Effects of sleep-promoting substance
on sleep-waking patterns of male rats. Rep Inst Med Dent
Eng 15:115-123
Inou6 S, Honda K (1986) Growth of rats exposed to far-infrared radiation. Zool Sci 3:731-732
Wolken JJ (1971) Invertebrate photoreceptors - A comparative
analysis. Academic Press, New York London, p 3
Yamazaki T (1987) Ensekigaisen ryoho no kagaku. Ningen-toRekishi-sha, Tokyo
Received July 2, 1986; revised June, 1987
Accepted October, 1987