“We must meet up before Christmas”, they said. “It’ll be fun,” they said. And just like that, every other day in your calendar between now and 31 December is accounted for.

For many of us, hectic socialising is part and parcel of the festive season; it’s as Christmassy as eating mince pies, singing carols and watching ‘Love Actually’ for the 15th time on ITV2.

But an entire month of late nights, often paired with large quantities of alcohol and stodgy, sugary food, can play havoc on our bedtime routine. What is festive party-going really doing to our sleep and will we regret it all come January? 

With mulled wine a-plenty (plus the work Christmas party to get through), drinking in moderation becomes a distant memory for many of us in December. But while alcohol can help us fall asleep faster, it also reduces the quality of our sleep, according to Alasdair Henry PhD, a researcher at Sleepio.  

“During the first half of the night as alcohol is being metabolised, more deep sleep is experienced than normal,” he explains. “However, during the second half of the night once the sedative effects have worn off and alcohol has been metabolised, sleep becomes lighter and more disturbed. This leads to more frequent awakenings and difficulty falling back to sleep.”

Alcohol can also interfere with sleep by altering our body’s ability to regulate temperature and making us need the loo throughout the night, Henry adds. On top of that, it’s known to increase snoring, our risk of sleep walking and exacerbate other sleep disorders, such as sleep apnoea, too.

The result is a grumpy cocktail of tiredness and irritability the next day. 

Even if you don’t drink, the lack of a regular sleep routine throughout December can take its toll on your physical and mental health. 

A recent study found “social jet lag’ is responsible for seriously messing up our body clocks. Put simply, when we stick to a sleep routine on weekdays, then throw it out of the window at the weekend, the effect on our bodies is similar to experiencing actual jet lag. Now imagine that amplified throughout December each time we alternate nights in with nights out. 

“An inconsistent schedule can interfere with the body clock and negatively impact how you feel during the day as the body clock works best with a consistent routine, and also make it harder to get a good night’s sleep at the appropriate time,” says Henry. 

In the short term, sleep disturbance can increase irritability during the day, make us less likely to engage with others and predispose us to dwelling on negative events, Henry explains. Long-term, it can “increase the risk of depression and anxiety and a number of chronic health conditions including diabetes, high blood pressure, heart disease and obesity”.

[Read More: How To Make Your Bedroom A Relaxing Sleep Haven]

For most of us though, a few late nights throughout December is nothing to worry about. Lisa Artis, sleep advisor at the Sleep Council, previously told HuffPost UK that a regular bedtime is especially advisable for people who are poor sleepers, as your body clock will tune in to that regular routine.

“But for people who are good sleepers, there’s no need to really overthink it and worry about it too much,” she said. “If you’re a good sleeper, the odd late night or the odd lie-in isn’t going to do any harm.”

If you do want to limit the impacts of poor sleep this winter, Henry recommends establishing good “sleep hygiene” habits such as keeping your bedroom cool, limiting screen time once in bed and getting plenty of outdoor light during the day, which helps regulate the body clock.

Passing on that third mulled wine wouldn’t go amiss, either.