**4. Variation of sleep architecture at high altitude**

Polysomnography (PSG) is the gold standard for investigating sleep architecture. However, the technical complexity and logistic demands had brought restriction on its utilization during altitude studies. Although there are several studies that suggest wrist actigraphy-derived data on total sleep time, sleep efficiency and sleep onset latency were similar to those of PSG [24]; actigraphy is insufficient in detecting sleep stage and breathing events.

Objective assessment of sleep architecture at altitude by electroencephalogram was first reported by Joern et al. in 1970 [25]. They found a near absence of stages 3 and 4 and a 50% reduction in rapid eye movement (REM) sleep and reported PB and arousals in one subject. A later study in 1975 confirmed a decrease in deeper sleep and increase in lighter sleep stages and brief arousals after ascending to an altitude of 4300 m at the Pikes Peak when compared to subjects at low altitude [26]. Subsequent studies have generally confirmed the shift at altitude toward lighter sleep stages, with a variable change in duration of REM sleep and increased awakenings associated with PB [27–30].

Alterations in objective sleep parameters have also been observed during acclimatization. A recent literature review on high-altitude sleep concludes that during rapid ascent to high altitude, there is a reduction in total sleep time, sleep efficiency, and deep sleep (stages 3 and 4) (in new nomenclature N3) and a significant increase in arousals and PB [31]. These variations are possibly high altitude dependent, and the effects tend to moderate with acclimatization [6]. Hypnograms of a partially acclimatized lowlander sleeping and a native Tibetan sleeping at high altitude are shown in **Figures 3** and **4**.

Although subjective sleep quality is impaired at high altitude, attempts to find a correlation between objective and subjective measures have failed to find a connection [24]. One study investigated 63 participants who completed a 3-hour flight from sea level to the South Pole (3200 m) and discovered no association between self-reported sleep quality and sleep efficiency, nocturnal oxygen saturation, and apnea/hypopnea index (AHI) obtained from PSG [32]. When assessed by LL score, there was no significant correlation of the subjective sleep measurement compared to sleep efficiency derived from PSG and actigraphy [24]. Another study investigated 165 young male soldiers stationed in Tibet Plateau (3800 m) for at least 3 months. In a multiple regression model adjusted for age, service time, body mass index, Epworth Sleepiness Scale, anxiety, and depression, sleep onset latency (b = 0.08, 95% CI: 0.01–0.15) and NREM latency (b = 0.011, 95% CI: 0.001–0.02) obtained from PSG were slightly positively correlated with global PSQI, while mean nocturnal SpO2 (b = 0.79, 95% CI: 1.35 to 0.23) and time in stage 3 + 4 sleep (b = 0.014, 95% CI: 0.001 to 0.028) was slightly negatively associated with global PSQI [12].

compared to Tibetans which is thought to be acquired and developed in adolescence [34]. Native Tibetans were reported to have higher maximal oxygen uptake, greater ventilation, and brisker hypoxic ventilatory responses to adapt to the hypoxic environment at high altitude and, therefore, to have a better-quality sleep than Han lowlanders [35] which may largely be attributed to genetic adaptations [36].

Few studies had compared sleep architecture between high-altitude dwellers and non-native highlanders. An elder study investigated the Sherpa highlanders dwelling above 3500 m. The Sherpas exhibited few PB with apnea due to low ventilatory sensitivity to hypoxia at 5300 m altitude [37]. A later study reported the sleep pattern of Peruvian Andeans situated at 4330 m altitude. Sleep architecture is closely resembling to normal of people at sea level with significant amount of NREM sleep and unimpaired REM sleep [38]. Contrary to the previous reports, a recent study surveyed sleep architecture of Peruvian highlanders living in Puno at 3825 m. The highlanders had a longer time in total sleep time and increased wakeafter-sleep onset and arousal index but decreased sleep efficiency, which suggest greater disturbances in sleep in highlanders compared with lowlanders [39].

As we mentioned above, acclimatization would help lowlanders to relieve sleep disturbance after ascending to high altitude. This could be supported by an earlier study which claimed over 3 days of acclimatization over 4559 m resulted in a partial recovery of sleep structure with increases in slow wave sleep and REM sleep and a

But little is known whether prolonged hypoxia may help to improve sleep architecture at high altitude. Animal studies showed that there was a 50% reduction in the proportion of slow wave sleep and loss of REM sleep when rats were chronically exposed to hypoxia environment simulating an altitude of 5000 m [41, 42]. A clinical study conducted in Shangri-La, which has an altitude of 3800 m, surveyed the differences in sleep architecture between native Tibetans' and Han lowlanders'stations for at least 3 months. After adjusted for the length of stay at altitude, significant differences in lower mean nocturnal SpO2 and shorter time in NREM sleep were determined in acclimatized lowlanders than the native Tibetans [12]. **Figure 5** indicates a decreased nocturnal artery oxygen of a 3-month

acclimatized lowlander. So, it is reasonable to conclude that the effect of prolonged acclimatization to hypoxia is limited in relieving hypoxemia and improving deep sleep which might be an explanation for the impaired cognition

*Decreasing in artery oxygen during sleep of a 24-year lowlander acclimatized for 3 months at 3800 altitude.*

**5.2 Sleep architecture in partially acclimatized lowlanders**

reduction in the arousal index [40].

*Sleep Disorder at High Altitude*

*DOI: http://dx.doi.org/10.5772/intechopen.86727*

brought about by poor sleep.

*The lowest SaO2 is 83% and the mean SaO2 is 92%.*

**Figure 5.**

**115**

#### **Figure 3.**

*Hypnogram of a 27-year-old young man sleeping acclimatized for 11 months at altitude of 3800 m. Frequent awake, less proportion of stage 4 and REM sleep might be observed.*

#### **Figure 4.**

*Hypnogram of a 25-year-old native Tibetan sleeping at altitude of 3800 m. There is sufficient time in stage 4 sleep and scarce REM sleep.*
