Associations of time-restricted eating with health-related quality of life and sleep in adults: a secondary analysis of two pre-post pilot studies

Sleep and health-related quality of life (HRQoL) are probably linked, as can be concluded from studies on insomnia [1]. Sleep as well as HRQoL are reported to improve with several days or weeks of therapeutic fasting [2]. This fasting for medical purposes is based on a severe form of caloric restriction to 250–500 kcal/day and is conducted in specialized fasting hospitals or integrative medical departments [3]. Fasting for a shorter period is also used for promoting health and preventing diseases by healthy applicants where it can be monitored by a fasting guide [3]. Therapeutic fasting should not be confused with very low calorie diets or formula diets that restrict energy intake up to 600–800 kcal/day. These diets are also maintained for several days or weeks but are aiming primarily at weight loss and are often subject to the so-called “Yo-Yo Effect”. That is a weight regain unintentionally after returning to the usual diet [4]. Continuous calorie restriction between 20 and 40% less than the average unrestricted food intake, however, has been tested in various trials in different animal species and, to date, seems to be the only non-genetic intervention to increase longevity [5]. In humans some concerns remain about possible negative side effects of calorie restriction [6] and it is understandable that studies in this regard are difficult to conduct. One of the rare studies is the Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy Phase 2 (CALERIE 2) investigation, a randomized controlled trial with 220 healthy non-obese participants. After 2 years of actually 12% calorie restriction, whereby the actual study goal was 25%, significant improvements in sleep and HRQoL were reported [6]. All these aforementioned fasting or calorie restrictive methods are either not feasible without medical or specially trained guides [3], carry the risk of increased weight gain after the end of the dietary phase [4], or place high demands on the adherence of those performing them [6].

Recently, another form of fasting has rapidly gained public interest. Intermittent fasting is the overarching term for a number of nutritional options with temporary calorie restriction. Days with reduced caloric intake alternate with days of normal or even increased intake. The schedule ranges from a daily alternation, over two preferably non-consecutive days, up to only 1 day per week. Intermittent fasting leads to comparable results as continuous calorie restriction in terms of weight loss, reduction in waist or hip circumference, fat mass or fat-free mass lost and ameliorated parameters related to glucose homeostasis [7]. A special form of intermittent fasting is the daily applied so-called time-restricted feeding (TRF), which still bears its origin from animal experiments in its name, but has meanwhile been renamed into “time-restricted eating” by some researchers in this field. TRF or time-restricted eating (TRE) allows ad libitum intake within a given time frame of 3–12 h each day, extending the nightly fast to 12–21 h as a result [8]. In animal studies, TRF was associated with weight loss, reductions in total cholesterol and triglycerides, improvements in glucose metabolism and markers of inflammation [8]. Interestingly, concerning humans, dropout rates in TRE studies were lower than in studies with other intermittent fasting regimes [8]. In a systematic review and meta-analysis of intermittent fasting diets including TRE, a significant decline in body mass index (BMI), fasting glucose, and homeostatic model assessment of insulin resistance (HOMA IR) as compared to controls was reported [9].

To date, only few studies of TRE have investigated sleep. After 16 weeks of TRE with a time frame for eating of 10–11 h, eight healthy overweight adults reported improved sleep [10]. Another study with 23 obese adults who followed an 8 h eating schedule for 12 weeks found no changes in sleep quality or duration [11]. However, we could not identify any TRE study that reported changes in HRQoL. A pilot study in ten sedentary older adults (≥ 65 years) found a tendency to a better quality of life, but this improvement was not statistically significant, possibly due to the small sample size [12].

In 2018 we conducted a pilot study with 63 healthy employees from the Ulm University where our primary outcome was adherence to and feasibility of TRE in adults in a working environment (unpublished material). We found an increase of HRQoL of 5.8 ± 12.4 on a visual analogue scale (EQ-5D VAS), independent of weight loss. In 2019 we conducted another pilot study with 40 abdominal obese patients in a general practitioners (GP) office [13]. Again, our primary goal was to test feasibility and adherence of patients in a GPs office. In the present article, we report results from a secondary data analysis of both studies with regard to changes in sleep quality and duration, and HRQoL after 3 months of TRE with a daily fasting goal of 15–16 h.

Both studies were conducted as pilot studies in a pre-post observational design. Details are already reported elsewhere [13]. The primary outcome for both studies was the proportion of days with reaching the fasting goal of ≥15 h out of the total number of days recorded per participant in the diary. According to the study protocol, secondary outcomes were, among others, changes in sleep quality and duration, and HRQoL between baseline and follow-up.

There were two dropouts in each study group. Reasons were personal overload, illness, occupational stress, and in one case unknown. Finally, data from 99 participants were available.

The baseline characteristics of the participants are shown in Table 1.

Differences between both study groups were significant for weight, waist circumference, BMI, WHtR, obesity, and abdominal obesity where participants from the Ulm University had lower values. On the other hand, participants from the GP’s office showed lower baseline values in HRQoL. At baseline, HRQoL and sleep quality showed a positive correlation (r = 0.254, n = 89, p = 0.016).

Three months of TRE correlated with changes in HRQoL by 7.8 ± 12.6 and sleep quality by 9.6 ± 13.9 points on visual analogue scales ranging from zero (worst quality) to 100 (best quality), but no extension of sleep duration was observed in two pilot studies with 99 participants. Interestingly, these parallel changes were independent from each other and independent from weight loss. Differences in HRQoL depended on mean fasting duration, baseline quality of sleep and HRQoL. It seems the longer the nightly fast, the better the HRQoL. Changes in sleep quality depended on baseline sleep quality and were correlated with HRQoL at follow-up. Sleep quality and HRQoL were positively correlated at baseline and follow-up, respectively. TRE was well accepted by participants with little side effects, which ameliorated in the course of the study [13].

HRQoL as a patient reported outcome is substantial for decisions about new treatments in the National Institute for Health and Care Excellence (NICE) of the National Health Service of the United Kingdom [16]. The preferred instrument for the assessment of self-rated HRQoL is the EQ-5D, from which the visual analogue scale (VAS) was taken for the TRE examinations presented here. The VAS is a validated instrument for assessing self-rated health [17]. The association between self-rated health and objective health status has been examined in a large population-based study indicating a consistent relationship [18]. HRQoL is a so-called “patient-reported outcome” (PRO) which is defined by the US Food and Drug Administration (FDA) as any statement about a patient’s health status made directly by the patient himself, without interpretation by a clinician or anyone else [19]. PROs are important because they make it possible to determine the effects of a treatment directly experienced by the patient beyond measurable clinical parameters. The changes in HRQoL associated with the TRE intervention was significant and independent from weight loss or reductions in abdominal obesity. With respect to the initial values, participants in the GPs office increased their HRQoL by 15.1%, those at the Ulm University by 7.8%. There is little information from research in the field of fasting concerning HRQoL, so this is one of the rare studies to add information to this topic.

Research on TRE and sleep is very rare. One study reported a significant increase in sleep satisfaction within 16 weeks of restricting daily eating duration to 10–11 h in eight healthy overweight individuals [10]. Another trial found good sleep quality throughout a 12 week intervention with a restricted eating time of 8 hours daily in 23 obese adults [11]. Participants in our study reported improved quality of sleep from initially 64.6 ± 15.6 to 74.2 ± 16.2 on a visual analogue scale ranging from zero to 100. Hence, restricting the time frame of eating seems to be associated with quality of sleep, but not, at least in our examination, sleep duration.

Therapeutic fasting may improve sleep [2], but some religious forms like Ramadan fasting may even reduce sleep duration and increase daytime sleepiness [20]. Sleep is of essential importance for our physical and mental health and poor sleep over a longer period increases the risk of health problems [21]. Sleep debt was identified to have an overall negative impact on metabolic and endocrine function many years ago [22], and was associated with an increased risk of diabetes in middle-aged men [23]. Newer studies indicate that sleep loss nowadays is, in parallel with the pandemic prevalence of obesity and type 2 diabetes, a specific problem of modern societies [24]. Interestingly, shift work, known for its disruption of circadian rhythmicity, was independently associated with poor sleeping quality and an increased risk of hypertension and type 2 diabetes in more than 25 thousand retired Chinese workers [25]. Not only the light/dark cycle but the feeding/fasting cycle affects the circadian system of the human metabolism and TRE may help to restore circadian rhythmicity as it mimics the way men ate ten thousands of years throughout the evolution [26].

TRE is primarily investigated in terms of its benefits for weight reduction and metabolic improvement. The participants in our study were able to reduce both their weight and waist circumference, although these reductions were more pronounced in patients in the family practice. TRE may be less suitable as a temporary measure of weight loss since this happens slowly, but as a permanent lifestyle change for longer-term normalization of the weight and metabolism. Beyond weight loss and improved quality of life and sleep, TRE may have further positive influences on health psychological determinants such as wellbeing, happiness and self-efficacy expectation (unpublished data).

TRE offers a low threshold intervention with the potential to increase the health-related quality of life and sleep quality in the adult population. It is suitable for primary care as well as for self-administration. The results give reason to implement further preferably randomized controlled trials to gain deeper insights into the benefits of a non-pharmaceutical intervention for better sleep and quality of life.