Increasingly, with the accelerating pace of the '90s, couples are living more complicated and demanding lives and experiencing chronic time crunches. And, increasingly, couples are complaining that the lack of time puts a big strain on their marriages - and their sex lives.

"Time is definitely a stress factor in our marriage," says one woman. "By the time we get everything done, who has the energy for just the two of us?"That's an intriguing question. These days, who does have the energy for "just the two of us?"

More often than not, it's the man, says Roberta Markowitz, a psychotherapist and author of an article called, "Today's Busy Woman's Biggest Complaint: `I'm Too Tired for Sex!!"' (Redbook, November 1987).

That women often don't have energy for sexual intimacy isn't surprising. Increasing numbers of women are juggling home, job and family, adding more and more to their schedules without deleting much, and suffering from debilitating chronic fatigue - a result of overload, overwork and overstress.

Consider the impact of fatigue on women. Relates one: "By the end of the day, not only is my body wiped out, but my brain is, too. My head is so buzzing with `don't forget' and `do this' and `do that,' that when I hit the pillow, I just want to zone out."

Relates another: "I'm so tired by evening that sometimes the idea of shifting the laundry from the washer to the dryer - to say nothing of folding - seems as daunting as running a marathon."

Now what can you infer about the love lives of these dead-tired women, Markowitz asks. You guessed it. They're zilch! But what about men? Don't they work hard, too? What about their fatigue? Why isn't exhaustion such an issue for them? And why do men say "I'm tired - but not TOO tired?"

Markowitz's answer? "Men's sexual desire simply is not affected by fatigue in the way a woman's is. A husband may be tired and yet still hunger for sex, while his tired wife hungers for nothing but sleep. To her, sex may suddenly seem like too much of a bother - or worse, it may actually seem objectionable, a turnoff."

Two tired people get into trouble when they don't understand this basic difference between men and women, says Markowitz. The woman may end up "feeling guilty and apologetic - after all, she is rejecting her husband - and both partners may come to suspect that the woman is using fatigue as an excuse to avoid sexual relations."

But that's usually not the case, Markowitz emphasizes. Unlike a man, a woman who succumbs to exhaustion is affected by a delicate interplay of both emotional and physical factors. Physically, when a woman is profoundly tired, "even the most gentle caress - a stroke on the back, a brush across the waist - can be annoying, uncomfortable."

Says one woman: "Sometimes when he pats me, it just makes me want to jump out of my skin. And it's embarrassing, because it seems like I'm overreacting. He's not putting pressure on me - he's just being sweet - and I'm acting like he's done something horrible."

An extraordinary feature about women is that they are blessed with bodies that are sensitive all over, says Markowitz. In lovemaking, this sensitivity can prove a lovely bonus. When fatigued, however, this sensitivity can prove to be a curse, because a woman may experience physical touch as intrusive and overstimulating when her brain is already short-circuiting.

In addition to feeling physically overwhelmed, a very tired woman may also feel emotionally overwhelmed, a condition that reduces her psychological defenses and leaves her feeling vulnerable. She may then erect barriers or walls to protect herself from the outside world.

Finally, by the end of the day, a woman may feel used up, devoured by others' demands. "Day in and day out her children tug at her, her employer is making demands on her, and then she comes home and there's her husband."

Under these circumstances, says Markowitz, "It's not surprising that a woman sometimes will see sex as simply another set of care-giving responsibilities."

The woman wants to do well by everyone, including her husband - she wants to be responsible, patient, giving, kind. But she can finally reach the point where she's thinking, "I've got to keep something for myself. If I don't, I'll have given all of myself away."

Without understanding that fatigue can be a genuine source of sexual difficulties, a couple can easily find themselves trapped in a downward marital spiral. Because she's exhausted, she says no. He feels rejected and angry and presses her. She becomes defensive and angry. The couple moves on to mutual accusations and demands, and over time, the marriage deteriorates.

So what can a couple do about this disconcerting state of affairs?

- First, discuss critical differences in sexual needs and "programming." Approach each other from the point of view that "we're different" rather than "you're the problem."

- Become what Markowitz calls "sexual pragmatists." Spontaneity is wonderful, but in this complicated day and age you need to be adaptable. The best way to get "shared time" is to plan ahead. Get on each other's lists! Also consider planning time for intimacy several evenings a week, which may help you both relax on "off" nights.

- Develop the non-sexual dimensions of intimacy in the relationship. Take time for talking, playing and being affectionate.

- Recognize you're living under unprecedented and swiftly changing cultural conditions that are putting mega stresses on couples and blowing many out of the water. Make sure, under these unprecedented conditions, that work and role distributions in the marriage are fair and that both partners have personal time in the evening. If you're a woman, give yourself permission to take that time!