Christmas is a time for giving and being merry - but it's also a time when your colleagues can morph into monsters.

The annual arrival of tinsel, mistletoe and jingle bells can transform a normally rational and well-mannered employee into another person entirely, warns a recruitment company.

Global recruitment company Robert Half has compiled a list of Christmas characters to make sure those who pick up unattractive traits during the silly season don't cross the line from festive to frustrating.

The Not-So-Secret Shopper - prone to spending excessive time shopping - is warned to reign in the spending and make sure it's done only outside of work hours.

While those who take the festive costumes too far - the Human Holiday Display - are advised to keep it in moderation.

Co-workers who use the annual Christmas party as an excuse to let loose - the Party Animal - should keep in mind it's a work function and act appropriately.

Then there's the worst of the lot - the Grinch - who acts as a Christmas fun-sponge.

Megan Alexander, general manager of Robert Half New Zealand, said they came up with the characters after talking to clients about what goes on at Christmas.

"When you work with people all day every day, you hear different horror stories ... people talk to us, because we're outside the firm."

Ms Alexander said people should also take the characters as a warning. "When you're in the office, there's protocols that you need to be aware of in terms of looking after your career. People don't take it that seriously and can have faux pas that actually end their career in certain companies and we've seen it," she said.

Psychologist Barry Kirker had some advice for those who became frustrated with their workmates who morphed into horrible Christmas versions of themselves.

"If an individual is getting annoyed with colleagues they can start to daydream about their upcoming vacation and how they will be away from the clowns in their office.

"They can also take pleasure in taking photos of colleagues in embarrassing situations. They can say to themselves Christmas is a fun time of year and it's good we can celebrate here whereas other places in the world do not."

They could also write Christmas cards for each of their colleagues and their boss and make "insightful comments" about how they see and feel about them.

This activity would stimulate them, shares good will and release frustration, he said.