Another sign of heatstroke is rigors (uncontrollable severe shaking) which I had once in Israel, another time in Greece. Because I am not good in high temperatures I have an accurate temperature gauge fitted to my bike with the sensor in the shade away from heat.

My take is as follows:
20C typical English summer day
24C English heatwave
28C lovely--warm enough to ride in T-shirt
32C about as hot as I like it. Take lots of water, sugar, salt
36C getting silly, especially mid summer, head for a swimming pool
40C TURN BACK, head for the coast or mountains

Once it's over 40C you can no longer raise your visor to get air flow over the face as it's like facing a hair dryer, so you have to ride with the visor closed. These are shade temperatures and you have to understand that 36C in October isn't the same as 36C in June/July when the effect of the sun is far more severe.

I also was in the same area last week. I had been planning to do the route Foum Zguid - M'hamid - Taouz - Merzouga - Rissani with Alfie, my son-in-law. He was flying in to Marrakech, so before he was due to arrive I was checking out some of the pistes around Zagora. But the conditions were insane with highs of 41C and overnight lows of 28C. Coupled with absolutely zero shade... I saw two groups of Spanish riders on pogos, but no northern Europeans.

The only way to deal with heat like this is to start real early (I started pre-dawn once) and then find somewhere with a pool to handle the heat of the day which is from 2pm to 6pm. But when it doesn't cool down overnight your body can't handle it. The high overnight temperature was the main cause of the 14,000 heat-related deaths in France in 2003.

Another trick, if you have access to sufficient water, is to soak your clothing in water. This particularly helps if you are riding, even slowly, but is less effective if you are stuck. I have been doing this for the last few days as it is still hot in Morocco, it was 36C yesterday leaving Marrakech. But you can't do that if you are with little water in the sand dunes.

Because of the heat I decided that when Alfie arrived we would discard the idea of the southern sandy pistes and after visiting Ouarzazate we would go to the Todra then into the High Atlas at Imilchil and then head west to check out the connecting pistes across to Anergui and then on to the Cathedral. Nevertheless the 39C temperatures of Ouarzazate took their toll and on the road to Boumalne du Dades Alfie was complaining of headaches. He was fairly well hydrated but we stopped for sugary mint tea and I mixed up three rehydration sachets and more salt (really essential as salt encourages the body to keep hold of the water rather than passing it straight through).

After we got to the Todra Alfie could go no further--bad headache, lethargy, nausea. So even though it was only about 2pm I organised a room in Kasbah les Roches and off he went to sleep in the cool dark room. Later that evening he was much better and we cooled down in the freezing cold spring water of the Todra river and once we got used to the cold water we spent 30 minutes immersed in the stream cooling down our core body temperature.

Icy cold water of the Todra - YouTube

Then more helpings of salt with the evening meal and lots of hydration. After that, the altitude did the trick as the next overnight stop was at 5C! And after finding the highest navigable tizi in Morocco it was me who was suffering, this time from altitude effects with slurred speech and running out of breath.

Exped Mira II tent - YouTube

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"For sheer delight there is nothing like altitude; it gives one the thrill of adventure
and enlarges the world in which you live," Irving Mather (1892-1966)