From time to time I come across cabin crew that have no idea where the air which they breath in the cabin comes from. Some believe that the flight deck crew give themselves all the "Oxygen" and the leftovers are for the cabin. This is obviously hogwash and we all breath the same air (which is why no smoking should be allowed on aeroplanes, but that is another war.....).
No-one seems to explain the basic technical workings of an airliner to cabin crew during their ab-initio training. So for the benefit of those who would like to know, here is a very short (or maybe not) simplified explaination.
When a airliner is on the ground with the doors open, the pressure inside the cabin is equal to the pressure outside. No sh!t I hear you say. Bear with me.
Once airborne the pressure outside the cabin reduces as the aircraft climbs, so to enable everyone to breath air is vented from the engines via PACKS into the cabin (called BLEED AIR). The temperature of this air is high when exiting the engines and it has to be cooled before being introduced to the cabin. By adjusting the temperature of this air, we have air conditioning.
Pressurisation is achieved by trapping air inside the cabin and allowing only a little to escape for the purpose of ventilation and cabin pressure differential regulation. Let me explain the latter. The airframe structure can only sustain a certain amount of pressure from the inside before it POPS (much like a baloon), so to keep a cabin altitude of 1000 feet for example the aircraft would only be able to climb to about 15000'. Should it continue to climb while maintaining the 1000 feet cabin altitude would mean eventual structural failiure. Increasing the altitude of the aircraft therefore requires increasing the altitude of the cabin. The engines still "pump" in the same amount of air, but the excess air is now vented via the outflow valves which keep this differential at a favourable and safe level.
All this means that at 35000' the cabin altitude would normaly be in the region of 5000 - 6000 feet depending on aircraft type.
Longer flights (especially on B777) normally end up at 39000 - 41000 feet. As the cabin altitude increases the light headness that you feel might be a mild case of hypoxia attributed to the fact that you are physically active (running around the cabin) at this cabin altitude. Like normally exercising at sea level and then taking a jog in then mountains - you simply don't perform as well. Physically unfit people will be more prone to feeling these effects.
Being on a B777 or a BAE146 makes very little difference (unless you work for a company that has humidifiers on their aircraft - that helps on long flights for dryness). Its the length of time that you are exposed to these high altitudes and how active you are that counts.
Hope this helps.