Sleep is important to human and athletic performance. Fullagar, Skorski (1) reviewed the effects of sleep deprivation and sleep restriction based on athletes’ physiological response, cognitive performance, and mood response. Thus, sleep quality is closely related to athletes’ optimal performance. In this regard, identifying factors that enhance sleep quality is a critical issue. Previous studies have developed different approaches to improving individuals’ sleep quality, such as exercise, mindfulness (2), and cognitive behavioral therapy (3). One novel strategy that has received attention recently is gratitude intervention. Emmons and McCullough (4) found that counting blessings increased hours of sleep in patients with neuromuscular disease. In addition, Digdon and Koble (5) found that a gratitude intervention improved cognitive and somatic pre-sleep arousal and worry and increased total sleep time and sleep quality compared to baseline.
Dispositional gratitude is viewed as a moral affect and not just a personality trait. Weiner (6) explained that gratitude has two steps. People obtain happiness through a positive outcome and then attribute the happiness to external sources, such as others or the surroundings. This kind of happiness is viewed as gratitude. One study revealed that grateful people are prone to mood stability and tend to exhibit more positive outcomes (4). Hence, some researchers developed a gratitude intervention and found that it can lead to increases in well-being, positive affect, and sleep quality (4). For example, researchers found that dispositional gratitude improved sleep quality because it mediated pre-sleep cognitions by promoting positive pre-sleep cognitions and diminishing negative pre-sleep cognitions to protect sleep quality. (7).
To the best of our knowledge, three studies have directly manipulated gratitude to investigate its effect on individuals’ sleep quality. Emmons and McCullough (2003) were the first to use counting blessings to manipulate gratitude, and they found, based on self-report data, that it increased hours of sleep in patients with neuromuscular disease. Furthermore, Jackowska, Brown (8) measured sleep by the Pittsburgh Sleep Quality Index and added several biological measures and found that a brief gratitude intervention could improve sleep quality and decrease diastolic blood pressure. Lastly, Digdon and Koble (5) also used self-report questionnaires and showed that a gratitude intervention resulted in significant changes in pre-sleep arousal, bedtime worry, and sleep quality compared to baseline levels but no significant changes in bedtime thinking, worry, anxiety, or sleep onset latency.
As reviewed above, previous studies have provided evidence demonstrating that gratitude improves an individual's sleep quality. However, there are two obvious weaknesses that need to be further investigated: there is no objective indicator to index the improvement of sleep quality, and the intervention has not been examined in a population that experiences periodic stress, such as athletes anticipating upcoming competitions. Therefore, we used EEG to collect objective indicators to eliminate recall bias, which we believe increases the internal validity of our study. These indicators are the EEG alpha (8–13 Hz) power ratio, EEG delta (0.5-4 Hz) power ratio, and EEG delta (0.5-4 Hz) power ratio in sleep stage N3. The EEG alpha power ratio represents the degree of wakefulness of a subject and is an index of whether the brain is at rest or not. During deep sleep, stage N3 consists of high-voltage (> 75 µV) and low-frequency (< 2 Hz) activity. When it is not in stage N3, the brain still needs to organize memories and information from daily life during sleep. In other words, the brain is only truly resting in stage N3. Therefore, the EEG delta (0.5-4 Hz) power ratio and EEG delta (0.5-4 Hz) power ratio in stage N3 can be used as indices of the deepness of sleep.
We applied a gratitude intervention immediately before the National Games to test the efficacy under extremely competitive conditions. The National Games is the highest and largest comprehensive sports competition in Taiwan, which is held every two years. The authors believe that sport provides the ideal conditions for examining the positive effect of a gratitude intervention because athletes participating in this event are top in their field, and the outcome of the competition determines their future career, whether this is a ticket to the Olympics or a guarantee of recruitment by a top university. It is conceivable that such a stressful situation would disturb sleep quality, especially as the day of the competition nears. Therefore, we chose to conduct research at this time point to examine our hypothesis.
In sum, the aim of the current study was to examine the positive effect of a gratitude intervention on elite athletes’ sleep quality. We conducted our experiment before an upcoming competition and adopted objective indicators of sleep quality to provide new insights for the gratitude literature.