Jay suggests that one should avoid publishing a new post in the night as advertisers have already exhausted their budgets and you are likely to run ads with very poor CPM rates:
Don’t post after dinner: Back to money for a second. If you use any form of pay-per-click ads on your site (AdSense, Yahoo, etc), I have found that posting after dinner time (6PM PST) results in poor paying ads. This is because advertisers reset their clocks at midnight and thus, the daily ad budgets reset. Therefore, posting at night allows your visitors to read your new content while low paying PPC ads are displayed after big spenders’ ad budgets have been depleted.
By posting in the morning, you get ads generated by advertisers with full daily budgets. Trust me, I have thoroughly tracked this and posts published in the morning are much more profitable. Link.
True but it also depends on your blog demographics – if you have a global audience, your visitors in US may see low paying ads due to the “midnight phenomenon” but remember that it’s noon here in India so the Indian visitors are likely to see high paying ads.