Sabratha was a city that once provided anchorage for Phoenician sailing vessels, was home to Greek mosaic craftsmen and was an export port for wild animals destined for Rome's coliseums.

It lies in splendid ruin beside the Mediterranean Sea just west of Libya's capital Tripoli. Unfortunately, on this - my second trip - I am seeing it while in the throes of a throbbing headache. I have no-one but myself to blame. Today is a fasting day in the Islamic world and my guide Mahmoud suggested I join him on the pre-dawn to dusk avoidance of all food and liquid (the fast also means abstaining from smoking and sex during these hours...neither of which, thankfully, will worry me today). I am keen to try both to understand better the process of fasting and to hopefully grasp a little of its spiritual significance.

The occasion is Eid-ul-Adha, the Moslem holy days that coincide with the Hajj or pilgrimage to Mecca. While millions of devout pilgrims gather at Mt Arafat, those not on the Hajj fast. There is however a slight hitch in Libya this year. For some reason, which no-one seems either able or willing to explain to me, Libya has declared while everyone else is in the Muslim world is marking the day of fasting, Libya would jump to the day after – the day of sacrifice. On this day families slaughter an animal in its prime – the family eat a third, a third is given to neighbours and other family members and a third is distributed among the poor. The day commemorates Abraham's willingness to obey God's word and sacrifice his son Ishmael. Just as Abraham was about to do so, a ram appeared in his son's place.

I started my fast just before the first prayer of the day at about 5.15am. There was no chance of being late...there are several mosques near our Tripoli hotel and on such an important morning it seemed the muezzins who make the call to prayer had amped up the volume. Although my stomach was decidedly not awake I drank a glass of water and ate a banana. That would be it until after 6pm.

When Mahmoud arrived at the hotel to collect me and my group he was looking unusually vexed. I couldn't believe it was food or liquid deprivation all ready. He explained throughout Libya there was now much confusion - whether to fast, or sacrifice the sheep. Some people wanted to follow what was happening in Mecca, while others were nervous that if they didn't celebrate the feast day they would be equally in the wrong. Mahmoud had vowed to fast so I decided it would be churlish now to nip up to the restaurant for coffee and some of the pastry chef's wonderful breakfast creations (most of us had become shamelessly addicted to his custard-filled é doubt we'd also pay for that when we got home and wanted to go swimming...).

By the time we reached the ruins of Sabratha I was feeling optimistic that a full day's sightseeing in the sun was going to be manageable...I felt just fine. Mahmoud handed us over to a local guide, saying he'd stay with the driver. I teased him that this translated as he'd have a sleep in the shade. I set off into the city and it was not until I start climbing the steps to the Antonine Temple that I realised I'd been a little optimistic.

There's no escaping the fact that I now have a pounding caffeine-deprivation headache. However, through half-shut eyes I can still appreciate the turquoise and lapis blues of the Mediterranean. The pillars of the Temple of Dionysius, built nearly 1800 years ago, stand before the sea, effortlessly photogenic.

An elderly Frenchman is photographing a young girl, inexplicably wearing tartan trousers who is perching on one of the truncated columns. She waves her legs in the air and thrusts pert breasts skywards. The locals look somewhat bemused. "I do hope he's her father," says one of my group. "And that makes it better somehow?" someone else remarks, staring in fascination.

We check out the oil press which once produced oil for export to the rest of the Roman empire and sit on the comfortable marble latrine seats. Toilet visits were a communal event and there would have been no worries about a cold bottom for those with the money - they'd simply send in their servants first to warm up a place.

By the time we reach Sabratha's highlight, its Roman theatre and the largest in Africa, I have to admit defeat. Built in the second century AD, the theatre was partly rebuilt in the 1920 and today its three-tiered richly carved facade is one of the most spectacular to be found in any Roman site in the world. Once, 5000 people could attend performances here. There were even galleries of shops underneath the theatre and special seating for VIPs. But all I want to know about is a cappuccino.

I head back to the bus where Mahmoud is reclining, hand over eyes, in the front seat. He groans when I wake him up. "I have a terrible coffee headache," he says. I am uncharitably delighted to find that he, who has been fasting regularly for most of his life, for Ramadan and other occasions, is also suffering.

"The secret is to have a coffee at about 5am," he says.

"Thank you for telling me now," I reply.

"Well, if it makes you feel better I forgot to have one too."

While the group eats lunch at a nearby restaurant Mahmoud and I retire to the guides' room and stretch out on the floor cushions and doze. I wake up only when our driver arrives, a cup of steaming coffee in his hand. Not fasting himself he cruelly wafts the paper cup under my nose.

"You need to control your mind – tell it you are fine," Mahmoud explains to me. "That is one of the points of fasting – controlling your desires. God's angels need neither food nor water so fasting is one way to come closer to God as they are."

I tell myself I am not feeling nauseous from the headache and it works. I might not be that much closer to the angels yet but it's a start.

As we drive towards the airport for a flight to the Sahara I reflect there is a sense of common purpose in fasting...knowing that millions of other people are doing and feeling just the same thing. This reinforces a sense of community and maybe even creates some elation at temporarily conquering (if not entirely) the demands of the flesh.

As the sun began to set into a dusty sky, Mahmoud and I order coffees, juice and share out the Panadol. When he judges the time is right Mahmoud prays for both of us and we reach for the juice....