If those long days of sun, sand and sleep-ins are but a distant memory, and you have the back-to-work blues, David Bilkey might have a fix for you.
The University of Otago psychology department professor and Brain Health Research Centre director says the back-to-work blues are quite normal - and quite avoidable.
One of his areas of expertise is circadian rhythm - a 24-hour cycle in the physiological processes of living beings which Prof Bilkey believes plays a major part in people's mental health when they return to work.
"It's your body clock. The neural systems that controls when we wake up and feel alert in the morning, and when we feel tired at night, that's circadian cycle.
"And, of course, one of the things that we do when we go on holiday is we tend to sleep in in the morning, and perhaps go to bed later than we usually would.
"People often don't realise that when you turn around and have to get up at 6.30am or 7am to go back to work, you're having to reset your body clock again.''
Prof Bilkey said it was difficult to shift a person's body clock by more than an hour every day.
"If you're used to waking up at 8am during your holiday, and then you suddenly have to shift to waking up at 6am, you're going to find that's quite difficult to do.
"As a result, you'll feel pretty tired in the morning, even at 8am.
"And if you're operating machinery or driving, you may just want to be a little bit careful about that.
"You should be aware that you're not going to be as alert for the first couple of hours of your work day.''
Prof Bilkey said it could take at least a couple of days for the human body clock to reset, and during that time it was normal for people to feel a little blue or cranky.
"It's just part of the change of routine. Part of it is the result of the change in your sleep/wake cycle, and part of it is just the increased pressure and stress that is often associated with your job and the contrast between your holiday life and your work life.''