Sheba has often told me that she thinks there's a rhythm to married life, an ebb and flow in the pleasure that a couple take in one another. The rhythm varies from couple to couple, she says. For some couples, the see-saw of affections takes place over a week. For others, the cycle is lunar. But all couples sense this about their life together - the way in which their interest in one another builds up and recedes. The happiest couples are the ones whose cycles interact in such a way that when one of them is feeling jaded, the other is ardent, and there is never a vacuum.