This is another one of those questions, like — Should I take a sleeping bag in summer time; is a rain jacket better than a poncho; should I take rest days; etc etc. The answer is hugely individual and depends on your own body rhythms.
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@rorerich, I travel from the US midwest and my flights are usually in the 18 or 19 hour range. For years, I always took a rest day upon arrival. One year, I decided to just bite the bullet and start walking on the day after arrival and found, much to my surprise, that it was PERFECT for me. Maybe not for you.
I find that the sleep I get the night of arrival in Spain is not normal, continuous, restful sleep but usually wakes me up in the middle of the night Spanish time. Tossing and turning for a few hours, and then getting up and going, being outside to experience the change in new sunlight patterns is the best way for me to acclimatize. Sometimes it’s a short first day (like the 12 km from Almería to Rioja), but sometimes I walk a normal day. It’s no different for me than walking after a sleepless night in an albergue full of snorers. In fact, I typically feel more rested!
There is no right answer here, and you can’t know how your body will respond till you try it. And like
@J Willhaus suggests, getting older can also affect your body’s internal clock, so you have to be flexible.
Whether you take a rest day or not, I have also found that getting to your starting point on that first day of arrival in Spain works best for me. I get to Madrid around 8 in the morning, and power on to make a connection on an internal flight or a train or bus to my starting point. I figure that the day of arrival in Spain is pretty much a lost day, so I try to use it to get my travel out of the way.
Buen camino, Laurie