Please be aware that caffeine and nicotine can trigger irregular heart rhythms in some people - and this may be what you're experiencing. Sleep apnea is a thought, but it usually occurs in REM (deep) sleep - and not when you're just falling off to sleep. If you are a long term smoker, the caffeine and nicotine may also be augmenting the potential for arrhythmias due to underlying heart disease. Congestive heart failure could present as you've described as well. You need to seek an evaluation from your doctor as soon as possible. This could possibly be something quite serious.....