From this post on
effects of quitting cigs and starting to vape:
"V4. Headache, racing pulse, inability to sleep
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Tachycardia and insomnia
A racing pulse and inability to sleep are symptoms of nicotine OD that may be experienced by those who have never before had any symptom of excess nicotine consumption. It is not clear if these symptoms are due to a batch of e-liquid that was considerably stronger than labeled, or an eventual reaction to excess nicotine. The solution might be to reduce the nic strength to a minimum sufficient to produce the required results (removal of cravings, adequate life functionality, stress relief, etc.) and no more.
Note that there is a huge range of tolerance to nicotine: some people report that 12mg liquid causes these types of symptoms for them, while others consume fairly large quantities of 36mg (or even higher) with no effects whatsoever. There are cases of users with 6mg liquid who reported that over-consumption was problematic, compared with a case of one user who vaped 60mg with no effects. There are many reports of people consuming 48mg in order to successfully convert to an e-cigarette. This appears to show that there is a factor-10 variance in nicotine tolerance between individuals - that is, some can tolerate a nicotine intake ten times higher than others.
As a general policy, it is probably best to reduce the nicotine strength to the minimum needed. It will often be impossible to equate this to the amount smoked as for some reason there is no direct relationship, for many/most people. Also, the nicotine strength of the liquid can be reduced over time, for most people."