Why Do People Make Love at Night?........ Reasons range from practical to romantic.

 
Most people make love when they go to bed, which is usually at night

 (1). Sleeping with someone is
thus a synonym for having sex. Why human sex and sleep are so intertwined remains mysterious.
Yet, there is no obvious reason why copulation under the covers at night is biologically optimal. Anthropologists note that sexual infidelity can occur opportunistically at any time of day, beginning with dawn. That is when hunter gatherers typically leave their huts to urinate. It is also when couples have quick trysts out of earshot of their sleeping spouses.
Such meetings may accomplish the biological job of fertilization but leave much to be desired from a variety of perspectives, physiological, social, psychological, and ethical.

Why Sex Before Sleep May Be Preferable
Researchers find that most marital sex occurs around bedtime. More than half of sexual encounters occur between the hours of 10 PM and 2 AM with a smaller additional peak at 6 AM, when couples are likely to be waking

.(2). Couples are more likely to have sex on weekend nights, suggesting that work schedules dictate patterns of sexual activity to some extent. Avoiding sex on work nights may help employees to feel better-rested the following day.
The strong circadian pattern of sexual activity has a fairly simple explanation in terms of availability, or opportunity. Married people are more likely to make love at the time they go to bed because they are available to each other.
Of course, that begs the question of why married couples generally sleep together in the first place. Even if this question is ignored, one is left with the problem of why sex is more common in the evening than in the morning.

Physiological Explanations
For most mammals, copulation is brief, although there are exceptions, like dogs, where mates remain joined due to coital lock that may serve to foil male competitors.
The brevity of mating does not interfere with fertilization, however. The same may not be true of upright walkers like humans, for whom gravity would tend to remove ejaculate from the reproductive tract. If so, lying down after sex may increase the chances of conception. Lying down together may also contribute to intimacy and sexual pleasure, which are thought to affect conception as well.

Relationship Explanations
That a couple sleep together has diverse implications for pair bonding and relationship strength. Of course there are many possible reasons for sleeping together that have little to do with intimacy as such. For instance, a couple is more effective at conserving heat during the cold of night, or winter, which has survival implications for indigenous peoples like the Inuit. Similarly, two pairs of ears are better than one when it comes to detecting night-time risks, such as attacks by wild animals or enemies.
On the face of it, though, lying down in close physical contact promotes intimacy by increasing oxytocin production. This is the “cuddling hormone” that promotes relationship strength for many mammals, including powerful mother-infant bonds
 (3).
Spending time in close proximity thus contributes to the strength of pair bonds—not just in people but in many other species of mammals, including the humble prairie vole in which this has been extensively studied

(4).

For pair-bonded species, a close relationship is critical for success in raising offspring. Indeed, females prefer to mate with their partner and may refuse to mate with unfamiliar males.

This pattern applies to humans who satisfy most of the criteria for being a pair-bonded species (3). In that context, it is understandable that sexual relations occur most often during bedtime hours if prolonged physical contact promotes feelings of closeness and intimacy.

Sources
1 Palmer, J. D., Udry, J. R., and Morris, N. M. (1982). Diurnal and weekly, but no rhythms in human copulation. Human Biology, 54, 111-121.
2 Refinetti, R. (2005). Time for sex: Nycthemeral distribution of human sexual behavior. Journal of Circadian Rhythms, DOI: 10.1186/1740-3391-3-4
3 Barber, N. (2002). The science of romance. Buffalo, NY: Prometheus.
4 Insel, R., and Hulihan, T. (1995). A gender-specific mechanism for pair bonding: Oxytocin and partner preference formation in monogamous voles. Behavioral Neuroscience, 109, 782-789.