It's estimated that on average, dogs sleep around 12-14 hours a day. Of course, many variables can influence how much sleep any individual dog gets and needs, including the activity of people and other dogs or nonhumans with whom they live, breed, gender, age, and the amount of exercise they get. We also now know that as dogs are sleeping, they're also learning.
As surprising as it seems, little is known about how prior experiences influence a dog's sleep. Stress is known to influence human sleep patterns and poor sleep itself can be stressful, so it's entirely reasonable to suspect that stress can influence how dogs sleep and perhaps also have cumulative effects on how moody they are.
While I never thought about how the dogs with whom I shared my home were affected by their having had a "good day" or a "bad day," their patterns of sleep showed variations and suggested that they sometimes had a bad night of sleep. Often they kept me awake, and when I got up in the morning they didn't respond with their usual enthusiasm. Once, one of the dogs, Mishka, was restless for two nights in a row. When she was awake she was moody and uncharacteristically irritable and didn't much appreciate either my intrusions into her day or those of her neighborhood dog friends. After a couple of nights she was back to her friendly and relaxed self.
Because of the lack of knowledge about how a bad day might influence how dogs sleep, I was pleased to learn of an essay by Bob Yirka that summarizes the findings of a new study by Hungarian researcher Anna Kis and her colleagues, "Sleep macrostructure is modulated by positive and negative social experience in adult pet dogs." (The entire research paper is available online and is well worth reading.)
Similar to the study in which it was discovered that dogs learn when they're sleeping, noninvasive EEG techniques were used. Dogs were exposed to positive experiences with their human and an unfamiliar individual, including being petted, playing tug-of-war or fetch, or being talked to. Negative experiences included being tied to a door and being left alone for two minutes, after which the human entered the room without greeting the dog or making eye contact. Then, an unfamiliar human entered the room and made direct eye contact with them without saying anything.1
Having a bad experience influences patterns of deep sleep
Overall, dogs who had experienced a negative experience fell to sleep faster than those who had a positive experience, but they also spent less time in deep sleep—EEG recordings showed that stressed dogs spent around 20 minutes more in rapid eye movement (REM) sleep, on average. (Also, dogs who were labeled as "conscientious" showed more non-REM sleep after positive experiences. Basically, they slept better than others.)
The researchers conclude that their study "provides the first direct evidence that emotional stimuli affect subsequent sleep physiology in dogs." The results of this study should alert humans that they need to pay more attention to their dog's sleep patterns and the possibility that a bad night of sleep can have an effect on their dog when they wake up.
1A few people asked me about the ethics of stressing the dogs. The researchers note that their "Research was carried out in accordance with the Hungarian regulations on animal experimentation and the Guidelines for the Use of Animals in Research described by the Association for the Study Animal Behaviour (ASAB). The Hungarian ‘Animal Experiments Scientific and Ethical Committee’ issued a statement (under the number PE/EA/853-2/2016) approving the non-invasive study experimental protocol. All owners volunteered to participate in the study."