With seasonal festivities well and truly under way, some are already fantasising about a dry January full of early nights.

But Dr Alex George, public health advocate and former Love Island contestant, revealed last week in a video on his YouTube channel that a sober night’s sleep isn’t necessarily all it is cut out to be – at least not in the early days of sobriety.

Two years after George gave up alcohol, he revealed that his sleep “got worse for about eight weeks before it got better”.

For the first two months, he said, he struggled to get to sleep, felt tired when he woke up and had “crazy dreams.”

His comments are an insight into the often mysterious and misunderstood affects of alcohol on sleep. While some people associate a nightcap with a good night’s rest, it is important to remember that “sedation is different from sleep”, according to Professor Russell Foster, head of the Sleep and Circadian Neuroscience Institute at the University of Oxford.

One of the main issues is how booze disrupts our nightly cycles or what some people call our ‘sleep homeostasis’.

Ordinarily, sleep occurs in five stages between waking and rapid eye movement (REM). Between those are stages of increasing depth, N1, N2 and N3 (where N stands for non-REM). N3 is also called slow-wave sleep (SWS or deep sleep) which makes up about 80% of our sleep.

A typical night’s sleep, if we can get one, consists of four to five cycles through these five stages. Importantly, each of these stages are associated with different biological processes, all of them essential for us to get the full benefits of rest.

As Professor Foster says, REM sleep is “when you have your most complex and vivid dreams. It’s also associated with emotional processing – it’s the body trying to make sense of a complicated world.”

Multiple studies have shown that, when consumed in moderate or high amounts, alcohol causes an overall reduction of total REM sleep, which can, in turn, lead to forgetfulness, anxiety and problems with concentration.

Alcohol also causes an increase in SWS in the first half of the night, according to the majority of studies, leading to a deeper sleep initially. This may sound like a good thing but can lead to disruptions as the night goes on, due to something called “homeostatic recovery”.

Essentially, if you lose certain elements of the cycle at one point in the night, your body and brain will try to make up for it at another.

And because alcohol can increase SWS in the first half of the night, the brain spends more time during the rest of the night in light, non-REM sleep – which you are much more likely to wake up from.

“The other issue which is often ignored is the role of alcohol in obstructive sleep apnoea (OSA) and snoring,” Foster says. “What alcohol does is relax the musculature of the throat and the neck, which means you’re much more likely to snore and to exacerbate OSA.”

All of this means that, even if you’re in bed for longer after a drink or two, the quality of sleep can be markedly reduced. But what happens when it’s more than a drink or two – and it’s happening regularly?

The long-term effects of alcohol and alcohol-dependence on our sleep are, unsurprisingly, largely thought to be an accumulation of many of the short-term effects above. The other is insomnia.

A 2018 study found that, among alcohol-dependent patients, two-thirds suffered from insomnia (compared to one-third of the healthy, adult Western population).

Exactly why this happens is still unclear, but prolonged disruption to sleep, poor sleep hygiene, and increased tolerance of the sedative effects of alcohol are likely to play a role.

What’s a little clearer is the important role that neurotransmitters – signalling molecules in the brain – play in alcohol’s effects on sleep, regardless of the level of intake.

“What we do know is alcohol is interfering with the neurotransmitter systems of the body and these are changing across the sleep cycle and within sleep,” says Foster.

Importantly, although these changes are often reversible, our neurotransmitters can take time to recalibrate. This is why it can take up to eight weeks of abstaining from alcohol, as it did for Dr George – potentially longer for others – for sleep to fully recover.

This period can include insomnia and, tragically for those trying to abstain, continued issues with sleep which can often be a cause for relapse.

In his recent video, Dr George also mentioned “crazy dreams” following sobriety. There’s a scientific explanation for this too: the “REM rebound effect”.

Like homeostatic recovery, when REM sleep is depleted for whatever reason, the brain compensates. As dreams are most likely to occur during REM sleep, studies show that those who have given up alcohol can find themselves having more vivid dreams. This effect is likely to occur on a smaller scale after a drink or two.

But quantity and timing are key.

Professor Foster reckons one glass of wine is likely to reduce sleep quality “by about 10 per cent” – but once you start feeling tipsy, “that will be 40 per cent”. He also says those percentages will be affected drastically by how close to bedtime your drinks are consumed; the closer to bedtime, the worse they are.

So if it’s a good night’s sleep you’re after, too many nightcaps are not your best bet.