This procedure sounds uncomfortable
What they did was take one day's temperature-rhythm of people before they got on their plane
(that's the dashed-line curve, superimposed on all the curves for comparison)
Those on the LEFT flew EAST for six time zones - those on the RIGHT flew WEST same distance
and then for eight days after the flight the solid daily curve shows how their bodies tried to get back in sync with local time.
Some interesting conclusions could be reached :-
1) However, the doctors suggest only that the Eastward flyers are still out of sync after 8 days (compared to Westward who are OK after 5 - 8 days) because their bodies had to speed up their rhythm while Westwards folk only have to slow down.
2) We say the graphs show the Eastward flight put more strain on their brains and bodies - Both flights were six time zones distance - about the Atlantic plus some - but the Westward folk kept sync. with the lead part of their daily rhythm curve, just losing a bit of performance later in the "day" - while Eastward travellers got their cellular activiity "pushed back."
(in fact from day 3 to day 5 they seem to suffer a feverish reaction)
see Fertility Page for the full story; `Perceptions' published warning; and subsequent Press / Airlines reactions.
And maybe jetlag2.html, jetlag3.txt, jetlag4.txt for background discussions.
Graph - From Klein, K.E, Wegmann, H.M & Hunt, B.I. (1972) Aerospace Medicine 43, 119-32.