In September 1966 John Lennon cut his hair in preparation for the role of Private Gripweed in the film film How I Won the War. Lennon’ hair had been growing steadily since 1962, and by 1965/6 was worn at below his collor. Here is a photo of Lennon from what must be the summer of 66, when the Beatles were in the process of recording Revolver:

http://fuckyeahrevolver.tumblr.com/post/187225088/john-lennon-revolver-session

Now here is a photo from Autumn 66, during the filming of How I Won the War. It is clear in this picture that Lennon has had his hair shorn:

http://www.pictorialpress.com/entry/how-i-won-the-war?tags=how_i_won_the_war&page=0

Lennon is almost 26 now and in an unhappy period of his life. He feels unfulfilled in both The Beatles and his marriage.

Sometime in late 1966 or early 67 Lennon grows a moustache. Lennon’ moustache is first noticeable in the sessions for Sergeant Peppers Lonely Hearts Club Band, which began immediately after the release of Revolver, in winter 1966. Here is a photo of Lennon in early 67 where his moustache growth is evident:

http://minnesota.publicradio.org/collections/special/columns/music_blog/assets_c/2009/09/beatles_moustaches.shtml

Lennon is almost 27 now. Note how the hair has grown out since the filming of How I Won the War in Autumn 66. By the summer of 1967 Lennon has shaved off his moustache. This is first noticed in The Beatles appearance on Our World, a broadcast in which they performed their new single All You Need is Love. One notable development in this footage is the growth of Lennons sideburns. Lennon’s sideburn growth continues throughout 1967 through to 1969. See the photo below for a shot of Lennon during the studio recordings The Beatles embarked between the release of Pepper and the start of The White Album. Note the growth of his hair and sideburns:

http://exquisitelyboredinnacogdoches.blogspot.co.uk/2009/12/hey-bulldog-yellow-submarine.html

Lennon continues to grow his hair throughout 1968 and into 1969, by which time it has reached shoulder length. He also keeps his sideburns, though they become hidden under his hair. Please see the picture below taken in early 1969 to assess the growth:

http://newtoryburchhandbags.blogspot.co.uk/2012/01/tribute-to-john-lennon.html

Sometime in early 1969 Lennon decides to stop shaving. His beard growth is evident at his marrige to Yoko in March and during the Bed In for peace in June. The growth reaches a peak at the end of the decade. See this picture from late summer 1969, when Lennon, at 28 has already told the rest of the band he will leave after the next album- Abbey Road:

http://globalgrind.com/channel/gossip/content/1898684/john-lennons-abbey-road-suit-and-15-other-celebrity-iconic-outfits-worth-buying-photos

By 1970 The Beatles are over and at some point early in the year Lennon, at 29, trims his beard and cuts his hair. My research suggests that this must be the first time he cut his hair since September 1966, when filming for How I Won the War began. Here he is on Top of the Pops, approximately February 1970. We all shine on:

http://www.youtube.com/watch?v=EqP3wT5lpa4