Today, the third Monday in January, is known as 'Blue Monday' - allegedly the most depressing day of the year.
The name was coined by psychologist Cliff Arnall in 2004.
Dr Sabina Brennan, health psychologist, neuroscientist and author of 'Beating the Brain Fog' has said Blue Monday was a PR stunt by Sky Travel in 2005 and a lot of people have jumped on the bandwagon to sell products to counteract the gloom.
Speaking on RTÉ's Morning Ireland, she said: "There is no denying that January can be a tough month for people for various reasons."
She said overindulging in food and alcohol and skimping on sleep over the Christmas break ends up making people feel lethargic and irritable.
Dr Brennan said a way to counteract that is to reintroduce routine and regular sleep patterns, eat healthy meals and try to resist the cravings built up over the holidays.
She said people should think about today as just another day in winter, "where you might be feeling the effects of the winter weather, darker days".
Dr Brennan said missing out on daylight and low levels of Vitamin D can affect our mood and make us feel sad and tired and reduce energy levels.
Read more:
RTÉ Brainstorm: Blue Monday: no, today isn't the most depressing day of the year
RTÉ Lifestyle: 6 ways to make January less blue
However, she said there is nothing particularly wrong in feeling that way "if you think of it as hibernating or as wintering, that our moods are a little lower".
She said people should focus on the things we can be grateful for such as the stretch in daylight hours which means we are emerging from that period.
