How Long is a Submarine Day?
Prior to 2012: At sea, a typical U.S. submarine day is 18 hours long, not 24. Crews are separated into 3 different watch sections with 6 hours on watch and 12 off. 
After 2012: In 2012 the Navy began to allow for 24-hour watch schedules for submarines by relaxing rules related to the 6-hour watch which had led to 18 hour days in practice. By 2014 24 hour schedules were mandated. This means although the fact about the Navy using 18 hour days was true, the traditional 18 hour day is no longer practiced. Instead, a 24 hour day with 8 on / 16 off is common and partial watches are allowed.
TIP: This article was written before the switch had fully been implemented. The changes have been reflected above, the rest of the article is about the 18 hour day.
Why Does the Navy Use 18 Hour Days on a Submarine?
The 18-hour day is a matter of practicality; it allows submarines to be on watch 24/7 with the space limitations of the submarine taken into account.
What Does Hours “Off” Mean?
A crew member’s “off” time is the time they are off the watch. They do a number of things during this time including drills, maintenance, and sleep.
Has it Always Been an 18-Hour Day?
Since the 13th century, maritime workers around the world used a 4 hour on, 8 hours off schedule. 
The 18-hour day was created in the 1960’s to address the fact that in modern times submariners were suffering from sleep deprivation on long patrols due to the need to do things like train and conduct drills on their 8-hour off period. Scientists are showing that 18-hour workdays have a positive effect on submariners. 
This video can help you understand the history of submarines. Understanding the history of submarines helps understand why keeping an 18-hour schedule can make a difference over the duration of patrols.
How Do We Define “a Day”?
When we say submarine days are 18-hour days we aren’t referring to the definition of a day as 24 hours, we are referring to the definition of a day in a more broad sense and as a workday or schedule. We are referring to the way a day is experienced by a given group of people, rather than a day as a relation to the sun. 
The 18-hour day can also be referred to as a watch system or watch schedule.
Do Other Countries Use Different Watch Systems?
Other countries use different variations of the watch schedule to accomplish the same thing as the U.S. Navy’s 18-hour days. See a list of watch systems here.
yes it helps people soooo FACT
No longer a FACT! I was on subs in the 70’s/80’s and indeed the workday was 18 hours – – which WAS somewhat hard to adapt to. My understanding was that this changed in 2016 to a 24 hour day. Check your facts.
Thank you for the insight, will look into it!
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