“Sleep is arguably the single most important thing we do every day for normal physical and mental health, and we’re all sleeping about one hour less than our counterparts did in the 1950s.” That’s the view of consultant neurologist Dr Kirstie Anderson, of the Newcastle Regional Sleep Service, and one of the UK’s top sleep experts.

Dr Anderson told The Irish Times that sleep is vital for maintaining normal immune function; recovering from muscle damage, for example, after exercise; ensuring your cells divide normally; consolidating memory; enhancing mental health; and helping to regulate temperature, weight and appetite. "And it's worth remembering," she adds, "that the total amount of sleep we require changes as we age, with the average adult over 45 years needing about seven rather than eight hours of sleep."

Sleep is not a state of unconsciousness. “During the night,” explains Dr Anderson, “you go through non-dream and dream sleep in 90-minute to two-hour cycles, with light, non-dream sleep in the first 30 to 40 minutes and then deeper so-called ‘slow-wave’ sleep. Adults spend about 20 to 25 per cent of the night in this deep slow-wave sleep, where your body is properly relaxed and from which it’s unpleasant to be woken.”

In the first study of its kind to investigate sleep-wake cycling (SWC) in newborns, researchers from University College Cork and Cork University Maternity Hospital recently reported in the journal Clinical Neurophysiology that EEG readings of healthy infants taken within 36 hours of birth demonstrate well-developed SWC, dominated by active sleep. Interestingly, they state that "[I]n comparison with infants born by vaginal delivery or emergency caesarean section (CS), infants delivered by elective CS before labour demonstrate SWC characteristics, which may be reflective of a lower stress level during the birth process."

Anderson emphasises that sleep is an automatic action, like breathing, and insomnia comes from paying too much attention to something we shouldn't have to think about: "To quote from Gillian Flynn's novel Gone Girl, 'sleep is like a cat – it only comes when you ignore it!'"

Irish researchers are addressing other aspects of disordered sleep. Dietician Dr Conor Kerley completed his PhD in the sleep and respiratory department at Connolly Hospital, Blanchardstown and is lead author of a recent study in the prestigious medical journal Sleep on the role of vitamin D in adults with the obstructive sleep apnea syndrome (OSAS). What is OSAS? Kerley told The Irish Times that it's a common yet under-recognised problem in Ireland and most of the world: "In OSAS the airways close or narrow repeatedly during sleep, decreasing oxygen levels in the blood and causing frequent wakening. If untreated, OSAS can have serious consequences, leading to increased inflammation and an increased risk of heart disease and diabetes."

Up to 70 per cent of OSAS cases, says Kerley, are associated with being overweight. “However,” he adds, “OSAS can occur in those with a healthy weight too.” But why study vitamin D in OSAS? “Because vitamin D has been associated with many OSAS symptoms, my research group conducted a detailed study of vitamin D status in OSAS, finding that of 106 adults living in Dublin with OSAS, all but one person had low vitamin D levels. We also found that those with the lowest vitamin D had the most severe OSAS and the worst heart function.”

Would a vitamin D supplement solve the problem? "Our research group," explains Kerley, "conducted the first ever study of vitamin D supplementation in OSAS. Although our study was small, we noticed vitamin D supplements resulted in decreased cholesterol and inflammation with potential benefits in the long term. This research was published in the Journal of Clinical Sleep Medicine. It's too early to recommend supplements for everybody but the idea is promising. I advise anybody who's concerned about OSAS to be assessed by a sleep physician and to get an overnight sleep study. Anybody with OSAS should maintain a healthy weight, undertake regular exercise and consider getting a vitamin D blood test."

Dr Liam Doherty, Consultant Physician in Respiratory and General Internal Medicine at Bon Secours Hospital, Cork , has a specialist interest in Sleep Medicine. He told The Irish Times that in general, obese patients suffer from poor sleep for a variety of reasons: "A bigger size means a more cramped, uncomfortable bed; obesity is associated with osteoarthritis, and painful joints (hips,knees) disturb sleep; and a restricted night's sleep causes hunger. Hunger in turn, promotes overeating, leading to obesity. Late night feeding, particularly high-calorie food, causes further disruption to sleep."

Obesity, says Doherty, leads to low self-esteem which may develop into anxiety and/or depression. “Inability to fall asleep,” he explains, “or recurrent arousals from sleep, ie insomnia, are hallmark findings of anxiety or depression. Unfortunately, anti-depressants or the medications used to treat anxiety can also affect normal sleep architecture and worsen sleep patterns.”

The most important medical cause of poor sleep in obese patients is OSAS, says Doherty, and its three characteristic symptoms are heavy snoring, nocturnal breathing irregularities, and excessive daytime sleepiness. "The heavier you are," he says, "the more likely you are to have OSAS. In my paper Bariatric Surgery and its positive impact on sleep, published in the Irish Medical Journal, we reported that not only did weight loss successfully treat OSAS, but it also improved daytime alertness in patients without OSAS. It appears that obesity disturbs sleep, and weight loss improves sleep quality."

Dr Anderson offers further thoughts on sleep: “Telephones and gadgets such as fitbits are for days, not nights; if you wake refreshed, you’ve had enough sleep; exercise is good for brains and bodies, and regular light exercise as part of a daytime or early-evening routine, undertaken more than three hours before bedtime is helpful; and although we sleep a little less as we age, protecting our time in bed is important for long-term health.”

As Thomas Dekker wrote in The Guls Horne Book (1609): "Sleep is that golden chain that ties health and our bodies together."