Researchers at Surrey University inform us that British women get significantly less sleep than men. This may not come as too much of a surprise bearing in mind that it is usually mums who provide the bulk of childcare duties in the night. However, it turns out that a more persistent cause of wakefulness in women is not children, but grown-ups. The study identified the fidgeting of their sleeping partners throughout the night as a frequent factor in female insomnia, saying that the sleep shortage that often ensues can heighten stress and cause conflict in relationships.

A common underlying feature in night-time fidgeting is a condition known as restless legs syndrome (RLS), characterised by an uncomfortable sensation in the legs which gives rise to an overwhelming urge to move them. This condition overlaps with another known as periodic limb movements during sleep (PLMS), in which involuntary jerks of the muscles occur at night. There is evidence that caffeine can worsen or trigger RLS symptoms, and my experience in practice is that this seems to be true of PLMS, too. Those suffering from one or both of these conditions may benefit from a reduction or elimination of coffee, tea and caffeinated soft drinks from their diets.

Another strategy that may help is to boost levels of magnesium, which is crucial to muscle function. A deficiency in the nutrient does seem to be a common underlying factor in RLS and PLMS. Other symptoms of magnesium deficit include cramp and muscle twitching. I find that in sufferers of either of these symptoms, low levels of magnesium are likely. You can increase your intake of magnesium by eating more nuts and seeds. And I recommend supplementing with 350-500mg of magnesium each day, though the full benefit may not be seen for two or three months.

Another nutrient commonly deficient in sufferers of RLS and PLMS is iron. This is important for the production of the brain chemical dopamine, a deficiency of which can cause disturbance in body movement - especially in vegans, vegetarians and women of child-bearing age. Iron levels are best assessed by measuring the level of ferritin in the blood. In practice, giving additional iron to those with ferritin levels that are low or on the low side can help to relieve the symptoms of RLS and PLMS. My experience is that for nocturnal fidgeters and their partners, nutritional therapies can be the dream ticket.

Dear John

In a recent article on depression you recommended St John's Wort at a dose of 300mg, three times a day. But most of the instructions I've seen recommend a daily dose of about 350mg. Could you clarify?
Maggie

Two of St John's Wort's active ingredients are believed to be hypericin and pseudohypericin, together referred to as 'total hypericin' (TH). Clinical experience and some studies suggest that the effective dose of TH for mild to moderate depression is 1.8mg to 2.7mg per day. St John's Wort supplements contain about 0.2 per cent of TH. This means a total daily dose of 900mg will provide about 1.8mg of TH. The study I highlighted (in which St John's Wort was found to be as effective as the antidepressant paroxetine in treating severe depression) employed 900mg of St John's Wort a day, though twice this dose was used in more resistant cases. While I recommend a total daily dose of 900mg, there is no reason why lower dosages can't be tried. These are, however, less likely to be effective in the treatment of depression.

Nutrition news

For some 2,000 years, oysters have been hailed for their aphrodisiac properties. In the past, it has been suggested that the oyster's reputation as a sexual stimulant may be related to its high content of zinc - low levels of which in the body have been associated with reduced libido in both men and women.

However, recent evidence suggests that the oyster may have a more direct impact on libido, as a result of some of its other constituents. American research has found that oysters contain the substances D-aspartic acid and N-methyl-D-aspartate (NMDA), both of which have the capacity to stimulate the production of testosterone - the hormone believed to have a major influence on the libido of both men and women.

It is not known whether downing half a dozen or so oysters would raise testosterone levels to a degree sufficient to have the desired effect. However, this research provides at least some validation for the much reputed aphrodisiac properties of this mollusc.

· If you have any issues you would like Dr Briffa to address in his column, please email him on john.briffa@observer.co.uk. Please note that Dr Briffa cannot enter into any correspondence. You can also visit www.drbriffa.com. Before following any recommendations in this column, you should consult your own medical adviser about any medical problems or special health conditions