Discussion
sleepiness, PVT, sleep disturbance, stress and increase sympathetic tone
By using a cross-over within-subjects design with 20 young men, directly comparing two different sleep interventions, we suggest with our data that shortened sleep (sleep restriction) increases sympathetic tone and decreases vagal tone during night causing increased autonomic stress, while fragmented sleep does not affect cardiac autonomic parameters.Laharnar et al. already showed, that there was no difference in objective sleep efficiency between the eight nights. Subjective sleep efficiency had lowest values after the intervention nights, but not significantly lower. They also claimed lower wake-times, less light sleep, less REM-sleep and more slow-wave-sleep during restriction night compared to fragmentation night. Regarding these, there was a recovery effect in in restriction week. Also PVT showed a recovery effect after restriction week. They concluded that restriction displayed a stronger sleep disturbance and a higher need for recovery than fragmentationOur results point up these previous results on the level of autonomic parameters towards a higher sympathetic activity during R compared to F.We could further show with our simplified hypotheses that light sleep stages (N1 and N2) were more affected, than deep sleep stages (N3) and REM sleep by sleep restriction.Analyzation of HR and SDNN showed that already the on the intervention following night with undisturbed sleep shows similar heart rates and SDNN values than the baseline night does. Therefore, we assume the impact of sleep restriction to be short-termed.HR was significant higher during light sleep stages of the restriction night compared to the corresponding baseline night which could show that this higher sympathetic activation is caused by the expectation of inefficient sleep. While participants were not blinded of the interventions, they were exactly informed of what to expect in both sleep intervention nights, sleep restriction as well as sleep fragmentation. Therefore, it is implausible that HR differences between the intervention nights were caused by expectations. To be on the safe side, we did the analysis , see “However, it is noteworthy that participants did not complete the normal four to five complete cycles of sleep in the sleep restriction night due to the shortened sleep. Here, participants were woken up after only five hours (after light off time), three hours earlier than during baseline and fragmentation nights. This had especially an effect on REM and deep sleep stages. During a night with undisturbed sleep, the amount of deep sleep decreases with each complete sleep cycle while the amount of REM sleep increases. There are also physiological changes of HR during a night when comparing subsequent cycles: with each completed sleep cycle, RR-interval gets longer, thus HR shifts towards a slower beat as in light sleep stagesNevertheless, the increased HR during restriction night indicates less vagal influence and therefore, a lack of regeneration and recovery in this intervention nightThe HRV analysis of the time-domain showed that SDNN during WASO times was significant smaller during restriction night than fragmentation or corresponding baseline night. As SDNN reflects total HRV, the prior results can be confirmed: sleep restriction disturbs HRV more than fragmentation. HRV reflects the organism’s ability to adapt internal functions such as heart rate to environmental stimuliWe conclude that sleep restriction, even if it is expected, seems to have more negative affect on the cardiac autonomic tone than an expected sleep fragmentation, and should therefore be avoided.While there are a lack of studies investigating fragmented sleep and comparing it to restricted sleep, there are studies comparing sleep restriction to undisturbed sleep. Here, our results are in line with those studies. Castro-Diehl et al. also showed that patients had an increased HR and decreased HF (as a marker of parasympathetic nervous system) in the sleep restriction night compared to an undisturbed night. They concluded that shortened sleep causes a decrease in cardiac parasympathetic activity and/or an increase in sympathetic toneOne study compared HRV in restricted as well as fragmented sleep with contradictory results. They found increased HR and also increased HF during fragmentation compared to restriction night in healthy and relatively young men with a mean age of 29.0 ± 3.1 years. They concluded that fragmented sleep affects heart rate and its parameters more than restriction in younger subjectsThere are certain limitations to the study. Probands were notably healthy and young respective middle aged, but they were chosen, because the experiment was planned to be repeated in space and astronauts are on average this age and of good health. Our results are not transferable to the large spectrum of patients.We did not include women because other variables like the menstrual phase influence heart rate and its variabilityWhile Lo et al.A possible limitation is also the kind of fragmentation: participants were woken up by switching on the light and had to complete a sleepiness scale. Then, the light was turned off again and participants were allowed to continue sleeping. Maybe this disturbance was not strong enough to interrupt the sleep so that we recorded greater results regarding sleep restriction.In our study, patients were not blinded to the interventions. They were informed about the study procedure and what to expect each night. This already can cause stress and increase sympathetic tone. Nevertheless, information was given concerning both interventions. Additionally, there was no difference in HRV immediately before falling asleep, so that the concern, that the expectation of a sleep intervention caused more stress, could be refuted.During baseline nights, more sensors were applied causing less comfort may increase sleep disturbance. Only one baseline was recorded, consequently, participants were not able to adjust to the new sleep environment. In the literature this can be found as a first-night effectA quite large number of statistical tests were performed, so the differences may be related to type-I-error. The calculation of sample size was based on SDNN values obtained in a study with sleep restrictionOur study was a prestudy for further experiments under isolation or in cosmos. Therefore, not only autonomic tone, but also the performance after a disturbed night must be investigated: the need for recovery, subjective well-being and analysis of the psychomotor vigilance test (PVT) can be read in Laharnar et al.