A “graveyard shift” is a night shift, usually between midnight and 6 a.m. There is no direct explanation of the origin of the term, but the darkness and quiet of night perhaps reminded some of a graveyard. “Graveyard shift” was printed in The Daily News (Denver, CO) on June 3, 1882. The earliest citations all involve mining operations.
 
A Seattle (WA) legend is that graves were moved in the middle of the night in February 1884, and that the cemetery workers coined the term “graveyard shift.” However, there are no contemporary citations of this event, and the term “graveyard shift” had already cited in print in newspapers from Colorado (June 1882) and Montana (January 1884).
 
“If you work at a cemetery, every shift is a graveyard shift” is a joke involving the term.
 
   
(Oxford English Dictionary)
graveyard shift  n. (see quots.).
1907   Collier’s 26 Jan. 14/1   From the saloons came the clink of the chips. For it was the ‘grave~yard gamblers’ shift… The small hours of the morning..are theirs.
1908   Sat. Evening Post 7 Nov. 27/2   A month later he and his fellows went on ‘graveyard’ shift. ‘Graveyard’ is the interval between twelve, midnight, and eight in the morning.
1965   ‘E. McBain’ Doll (1966) ii. 22   The afternoon shift is from four p.m. to midnight. And the graveyard shift is midnight to eight a.m.
     
3 June 1882, The Daily News (Denver, CO), “Mine and Smelter,” pg. 2, col. 1:
Following this at about one o’clock yesterday morning, about eleven men with Winchester rifles approached the Mineral Farm, the graveyard shift being then at work.
 
Chronicling America
3 January 1884, The Mineral Argus (Maiden, MT), “Buttons of Bullion,: pg. 4, col. 2:
The mixologist on the graveyard shift at The Bullion, had better beware of the P. I. A word to the wise is suf.
     
Google Books
9 August 1884, The Mining Record, “Mining Notes—Arizona,” pg. 86, col. 2:
THE MOWRY CO.—Have resumed work. On Monday afternoon superintendent Hammond received a telegram directing him to resume operations in the mine, and at 6 P. M. the graveyard shift went below, while the whistle sounded loud and long, confirming the good news.
 
Chronicling America
26 March 1888, Omaha (NE) Daily Bee, “Some Scenes in Leadville,” pg. 3, col. 2:
All of the principal gambling houses run three shifts of men for each game. The after midnight early morning run is called the grave yard shift, but why the name should be applied to those hours more than to any other group of hours, is not quite clear.
   
Google Books
August 1893, Fetter’s Southern Magazine, “Fields of Gold,” pg. 153:
The workmen are divided into three shifts of eight hours each, the last going on at eleven o’clock at night, facetiously called the “grave-yard shift.”
(Mines in Cripple Creek, CO.—ed.)
 
Chronicling America
1 February 1894, The Salt Lake Herald (Salt Lake City, UT), “In Railroad Circles,” pg. 6, col. 5:
The new men of the department have nearly all been put on the night, or graveyard shift, as they are without uniforms and would, therefore, fail to make the showing desired.
 
28 November 1995, Seattle (WA) Post-Intelligencer, “Term’s Origins Matter of Grave Concern” by Jon Hahn, pg. C2:
And for two summers, I worked full-time in a cemetery, where every shift was a graveyard shift.
(...)
Greg Marshall, a Graytop driver and Seattle cabby for more than 30 years, explained over morning coffee at the Puppy Club that “graveyard shift” originated when workers were moving bodies from an old Pioneer Square cemetery to Lake View Cemetery.
     
HistoryLink.org
Seattle Cemetery
By Laura Angotti Posted 3/16/1999
The Seattle Cemetery, located at the present (1999) site of Denny Park north of downtown, was Seattle’s first official municipal cemetery. The first burials in 1861(?) were bodies removed from other informal cemetery sites to make room for buildings. The Seattle Cemetery served the municipality from about 1861 until 1884. In 1884, some 223 burials were removed to other cemeteries, a process which produced some curious results.
(...)
Local legend has it that this incident is the origin of the term “graveyard shift” for work done during the middle of the night.
 
American Dialect Society listserv
Alternate origin story for “graveyard shift”
James Callan jabeca at DRIZZLE.COM
Sat Aug 13 17:04:11 UTC 2005
Last night I went on the Seattle Underworld Tour, the risqué, 21+ version of Seattle’s famous Underground Tour. Among the many anecdotes told was the story of how, when the first Seattle Cemetery was moved during the 1884 regrade project, bodies were moved in the middle of the night to avoid disturbing people or otherwise causing a scandal. And that, the guide asserted, is where the term “graveyard shift” came from.
 
I found one Web reference to the story on King County’s history site, historylink.org
(http://www.historylink.org/essays/output.cfm?file_id=969 ):
 
> Given the conditions under which Shorey was striving to make the
> removals, it would not be surprising if he had missed a few burials,
> and so he did. During the final regrading of Denny Hill, in which the
> land on which the Seattle Cemetery had stood was lowered about 60
> feet, several bodies were purportedly found, probably Indian graves,
> as the regraders washed away the hill. Because it would have disturbed
> people and possibly caused a scandal, it is said that these bodies
> were removed to some unspecified place during the middle of the night,
> when supposedly no one would notice. Local legend has it that this
> incident is the origin of the term “graveyard shift” for work done
> during the middle of the night.
 
OCLC WorldCat record
Graveyard shift
Author: Ralph S Singleton; William Dunn; John Esposito; David Andrews; Kelly Wolf; All authors
Publisher: Hollywood, CA : Paramount, [2017]
Edition/Format:   DVD video : NTSC color broadcast system : English
Summary:
When an abandoned textile mill is reopened, several employees meet mysterious deaths. The link between the killings: all occurred between the hours of eleven p.m. and seven a.m. The sadistic mill foreman has chosen a group to clean up the mill’s rat infested basement, but what the workers find is a subterranean maze of tunnels leading to the cemetery, and an unimaginable horror that comes alive in the dead of night.