The wide windows which are of sliding type tend to let in more light and hot air during daytime.
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2s coaches, you don't notice it, plus you get compensated with a good view. The Emergency window seats have no grill, and they can be really awesome during good weather.
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But...
more... in Sleeper class you will feel the heat since there's partition between compartments, eventually the hot air rises up and accumulates at the top. Upper berth travel can leave you in a semi-cooked state if you're tired & try to nap for a hour or two. In the evening, you get to experience more of the cool breeze as a compensation.