If you are not getting enough sleep, among other health issues caused by a lack of sleep, you probably have a low-sex marriage. 

Theresa DiDonato: "Sleep may be a vital ingredient for relationship health and well-being. Whether you’re looking to meet someone or have an established relationship, take care of yourself by fostering healthy sleeping habits. It may make a big difference in the quality of your most important relationships."
Source: Theresa DiDonato, Ph.D.

"Why Your Relationship Depends on a Good Night's Sleep." PsychologyToday.com. 4/25/2014.

Andrew Hough: "Scientists from the University of Chicago found men who get less than five hours sleep a night for a week or longer suffer have far less levels of testosterone than those who get a good night’s rest."
Source: Andrew Hough. "Lack Of Sleep 'Kills a Man's Sex Drive', Study Concludes." Telegraph.co.uk. 6/03/2011. 

In June, 2001, the Chartered Institute of Personnel and Development surveyed managers who worked more than 48 hours per week. The survey revealed that there are a great many married couples who are just too tired to have sex. 

A 2008 survey by the Centers for Disease Control and Prevention suggested that adults in the U.S. are not getting enough sleep. More articles appear every year such as Arianna Huffington's 2010 Sleep Challenge about people, especially women, being sleep deprived.  

Depressing Survey Findings

• Nearly 1/3 admitted that work-related tiredness caused their sex life to suffer.

• 14% reported a loss of, or reduced, libido.
• Most self-described workaholics who participated in the survey were male, middle-aged, and married.
• Average income was put at 32,000 pounds ($47,020).
• The respondents generally had spouses who also worked.
• 70% of partners reported their spouse was even too tired to talk.
• 54% of partners said their sex life was suffering due to their spouse's long hours at work.
• 43% admitted to being fed up with having to shoulder most of the household chores.
• 42% believed their long work hours was the cause of arguments at home.
• One in three shared feelings of guilt about "missing out on their children growing up."
• Many admitted to having made mistakes at work.
• More than half believed they didn't have balance in their lives.

What Can You Do If You Are Too Tired to Have Sex? 

• Talk with one another about the sexual problems in your marriage.
•  Schedule a sex appointment with your spouse.
•  Just do it.
•  Try to get a good night's sleep.
•  Follow the sleep hygiene tips provided by the CDC.

Laura Berman: "I actually just did a big study on intimacy and found that couples who kiss regularly and spontaneously not only have higher levels of intimacy, general intimacy, but also have decreased levels of stress and depression ... try just kissing your partner more and cuddling them more."
Source: Situation Transcript. "New Study Claims Sex Can Help Sleep Deprived." NBCNews.com.  2/13/2006.