What do I call someone who appears never to sleep, always responding to emails etc whatever the time of day?
I would call them vigilant:
- keenly watchful to detect danger; wary
- ever awake and alert; sleeplessly watchful.
The word conjures up images of guards ready to respond to danger or trouble.
Depending on context you may use ,"a type A guy" "a workaholic" or "an automaton"
type A personality Type A individuals are described as ambitious, rigidly organized, highly status conscious, can be sensitive, care for other people, are truthful, impatient, always try to help others, take on more than they can handle, want other people to get to the point, proactive, and obsessed with time management. People with Type A personalities are often high-achieving "workaholics" who multi-task, push themselves with deadlines, and hate both delays and ambivalence.
a workaholic is a person who chooses to work a lot : a person who is always working, thinking about work.
an automaton behaves or responds in a mechanical way, a person who acts mechanically or leads a routine monotonous life.
May I suggest indefatigable as referenced by Hilary Clinton's aura of being indefatigable which slipped yesterday when she had to cut short her attendance at the nation's annual New York commemoration to the victims of 9/11. She was caught by her Secret Service agents in mid-faint and taken to her daughter's apartment nearby. Secretary Clinton's physician, Dr. Lisa Bardack, issued a statement released by Mrs. Clinton's office confirming that she (Mrs. Clinton) had been "...diagnosed with pneumonia...She was put on antibiotics and advised to rest and modify her schedule."
The text in quotation marks reported by The Telegraph online 12 September 2016.
indefatigable: Able to work or continue for a very long time without becoming tired. (M-W)
As a person well-known by his colleagues for responding to emails 24/7, I offer the following:
workaholic: a person who compulsively works hard and long hours [Google]
Some might add an element of obsessive-compulsive behavior:
obsessive-compulsive: denoting or relating to a disorder in which a person feels compelled to perform certain meaningless actions repeatedly in order to alleviate obsessive fears or intrusive thoughts, typically resulting in severe disruption of daily life [Google]
I'd call them a night owl.
Fig. someone who stays up at night; someone who works at night
[McGraw-Hill Dictionary of American Idioms and Phrasal Verbs via The Free Dictionary]
Constantly, regularly, or habitually active or occupied: diligent.
An industrious worker.
Argus fits this description. See https://en.wikipedia.org/wiki/Argus_Panoptes
In the 5th century and later, Argus' wakeful alertness was explained for an increasingly literal culture as his having so many eyes that only a few of the eyes would sleep at a time: there were always eyes still awake.