A recent study attempted to answer these questions, though the results bring up a few questions of their own.
This study found senior men who reported having trouble sleeping were more likely to develop depression a few years down the line.
But when researchers objectively measured the men's quality of sleep, they had no increased risk of depression unless they had a few minor symptoms of depression at the start of the study.
The good news is that men who feel like they're getting good enough sleep – whether they actually are or aren't – don't appear to be at higher risk for depression.
"Seek help if you're depressed and sleeping poorly."
This study, led by Misti Paudel, MPH, of the Division of Epidemiology and Community Health at the University of Minnesota, aimed to find out if depression in older men was linked to waking up frequently in the night.
In the past, links found between depression and frequent awakenings were based on self-reported sleep that participants provided in questionnaires.
For this study, however, the researchers used both questionnaires and actigraphs to measure participants' sleep quality.
Actigraphs are devices used to measure a person's activity levels and can be used to estimate sleep quality and quantity.
The researchers tracked 2,510 men, aged 67 or older, for an average of 3.4 years.
None of the men had depression at the start of the study, but 4.6 percent of them (116 men) were depressed at the follow-up.
The researchers compared the self-reported sleep among all the men, taking into account those who had some symptoms of depression at the start of the study but not enough to qualify as being fully depressed.
The researchers also took into account other factors that might have influenced these men's risk of developing depression, such as age, educational level, other health conditions, cognitive function and lifestyle behaviors such as smoking, use of certain medications and amount of exercise.
As in past studies, the researchers found that men reporting poorer sleep in questionnaires were about 1.5 times more likely to be depressed at the follow-up.
Then, the researchers looked at other sleep measures and their relationship to depression, before accounting for any symptoms of depression at the start of the study.
The results showed that the men who had poorer sleep, took longer to fall asleep each night, woke up more often during the night or spent more time awake during those nighttime awakenings were anywhere from 1.5 to 1.9 times more likely to be depressed.
However, once the researchers took into account the men's symptoms of depression at the start of the study, this link between non-self-reported sleep measures and depression vanished.
There was also no link between depression and total sleep time or feeling excessively sleepy during the daytime among the men.
Therefore, only men who reported having poor sleep were more likely to be depressed several years later at follow-up.
Those who experienced various issues with quality of sleep, as measured by the actigraph devices, did not appear at higher risk of developing depression after their emotional symptoms at the beginning of the study were considered.
"We observed that greater burden of depressive symptoms at baseline and health-related factors partially explained the association between poor sleep quality and risk of depression at follow-up," the researchers wrote.
The researchers also suggested that it's possible that depression may influence sleep in the same way that sleep may influence depression. In other words, any possible relationship between the two may work both ways.
The evidence for that relationship is one of the important aspects of this study, said William Kohler, MD, a dailyRx expert and the director of the Florida Sleep Institute in Spring Hill, Florida.
"This study shows the reciprocal relationship between depression and sleep," Dr. Kohler said. "Poor sleep increases the likelihood of depression and depression increases the likelihood of poor sleep."
He said that these findings match up with those in past research, in which individuals already at risk for depression may see their depression increase after sleeping problems develop.
"Previous studies have shown that for people who have depression but have relatively good control of it, developing insomnia is a harbinger that the depression is about to worsen," Dr. Kohler said.
The study authors wrote that it's not clear, however, why the men who reported poor quality sleep would be more likely to be depressed even though those who actually, objectively experienced poor quality sleep were not more likely to be depressed unless they already had some depression symptoms initially.
This study was published in the July issue of the journal Sleep.
The research was funded by the National Heart, Lung and Blood Institute, the National Institutes of Health, the National Institute of Arthritis and Musculoskeletal and Skin Diseases, the National Institute on Aging, the National Center for Research Resources, the NIH Roadmap for Medical Research and the Minneapolis Veterans Affairs Medical Center.
One author has received a grant from Litebook, Inc. and has consulted for or advised Ferring Pharmaceuticals, GlaxoSmithKline, Merck, Neurocrine Biosciences, Pfizer, Respironics/Philips, Sanofi-Aventis and Somaxon.
Another author has received research support and equipment from Philips Respironics, ResMed Inc. and ResMed Foundation.
A third author has received research funding from Elan, SIENA, PRANA and Teva. No other potential conflicts of interest were reported.