QUESTION: I am sick of hearing of ideal weight. How can I tell just what my ideal weight is? I know that what's ideal for one may not be ideal for another of the same dimensions. Am I right? I am 5 feet, 2 inches, average build and 120 pounds. - Mrs. S.A.

ANSWER: Truly ideal body weight is one with the correct mix of lean and fat weight. There are ways to arrive at that for the individual, but it's far too complex and costly for practical purposes. Leave it for the serious athletes to wrestle with.It is always hard to make general rules for this, since an athletic woman, for example, can afford greater total weight if some of her extra weight is in muscle tissue, which it usually is. For most of us, though, a simple formula gets us close enough for comfort. A 5-foot woman of average build can consider herself ideal at 100 pounds, adding 5 pounds for each inch over that. For a man, ideal weight for 5 feet is 106 pounds, adding six pounds for each inch over that.

Those are good weight targets for just about anyone, any plus or minus allowances for frame size being quite small really. You might subtract a pound or two from the total for small frame and add a couple for large frame. You'll be in the ballpark.

QUESTION: I'm a female, 77, and in very good physical condition for that age, I'm told. I don't think I'll be able to say this for long though, doctor, if I don't start getting some sleep. It seems I never get a full night's sleep. Even a glass of wine before bedtime doesn't get me drowsy enough. I take it around 10 p.m. and there I am, up again at 3 a.m. After that, I wake, doze, wake, doze, until dawn. It is not fun. Please tell me, short of sleeping pills, how to cope with this nightmare. - Mrs. O.B.

ANSWER: For starters, you can quit that 10 p.m. glass of wine. Alcohol can produce sleep initially, but it also induces later arousals, such as you report.

Now, it is perfectly normal for a person your age to experience new sleep patterns. Sleep becomes shorter and lighter with age. Less time is spent in the deep-sleep phases of the sleep cycle. The total sleep need over 24 hours remains static, but the nighttime portion may decrease with age. In short, many elderly people try to fill up a night with night sleep they may not need. It's the reverse of trying to fit a foot into an undersized shoe.

You have to establish a new set time pattern for retiring and awakening. In a short time, you will come to terms with the new sleep ball game. Avoid daytime napping, especially after dinner. Stay away from caffeine, including chocolates and colas. If you have urinary frequency, avoid all pre-retiring drinks. I am mailing you the insomnia material, which may offer other hints for you. Other readers may order by writing: Dr. Donohue/No.30, P.O. Box 830, Gibbstown, NJ 08027-9909, enclosing a long, stamped (52 cents), self-addressed envelope and $2.

Dr. Donohue welcomes reader mail but regrets that, due to the tremendous volume received daily, he is unable to answer individual letters. Readers' questions are incorporated in his column whenever possible.