Why do most saints grow a beard and/or long hair ?
Apart from appearance, is there any other significance ?
Any spiritual significance such as change in the flow of prana/energy etc ?
When Yogananda went to visit Japan in 1916, he decided to cut his hair short. But in 1920, when he left for America, Sri Yukteswar expressed the wish that he keep his hair long. I am sure Sri Yukteswar’s reason wasn’t Yogananda’s pretty looks. So your question about a deeper yogic significance of long hair is legitimate.
But before getting into that, let me finish the story. Yogananda actually had quite a tough time with his long hair in America. He told numerous stories like the following one:
"…Another time was when I was going to Harvard University to see what it was like. I asked the conductor, 'Is this Harvard University?'
"He replied, 'Yes, Ma'am.'
"Another time in a Boston food show with my friends, Dr. and Mrs. Lewis, I was given something to eat and I asked where I could wash my hands. The man directed me to a place and when I went in first thing I see, ladies to the left and ladies to the right. They thought it was all right, but I knew it wasn’t. So I ran out. I went searching for the men’s place and finally asked one man where it was.
"He pointed, 'There it is.' He showed me the ladies' place again. I had a mind to say, 'I was there already.'
"But I asked, 'Where is the men’s place?' As I was going very happily in, out comes one man running at me and crying, 'Upstairs, upstairs!' I said, 'Shut up. I know what I am doing.' When I got in, fifteen men were washing their hands and they all flew out of there like birds. And I was left alone… So, the thing is this, everybody in America mistook me for a woman because every woman has long hair."
So now about your question. Yes, there is a deeper yogic significance to long hair.
The Autobiography of a Yogi teaches: "The spinal cord is like an upturned tree, with man’s hair as its roots, and afferent and efferent nerves as branches." Elsewhere Yogananda explains: "Like an upturned plant, man similarly absorbs through his hair electric currents helpful to the body."
Swami Kriyananda expands on this thought in his book Raja Yoga:
"Yogis say that long hair draws more energy to the brain. They describe the body as an inverted tree of which the spine is the trunk, the nervous system the branches, and the hair the roots. This is why many yogis let their hair grow long. When I was in India, I allowed my hair to grow until finally it reached half way down my back. I discovered with long hair that I was subject to fewer headaches than I had been before."
You may meditate on this interesting explanation by Yogananda: "The hair is more on the head because that is where the seat of energy is.
"Some yogis do not cut their hair but keep it long, to draw from the ether a greater quantity of cosmic rays. The reason for Samson’s having lost his superhuman strength when his hair was shorn by Delilah may well be that he had practiced certain yogic exercises that transform one’s hair into sensitive antennae to draw cosmic energy from the ether."
Yogananda even writes: "Scientists are finding that ladies are the stronger sex because of the long hair."
Maybe men, then, would do well to change hair fashion!
About beards: Yogananda, on the ship traveling over to America in 1920, also had a "long beard," but his travel companion convinced him that both long hair and a beard would be too wild for Americans. So Yogananda "was willing to give in" and sacrifice his beard. He later also asked all his male disciples to shave off their beards, except Swami Kriyananda. "I don’t want my boys looking like wild men!" One "wild man" in the crowd was obviously enough.
Wild or not, long hair is definitively a good yogic tradition.