The short answer
Flying east is harder than flying west for almost everyone. It's not psychological. It's biological. Your body's internal clock doesn't run on a precise 24-hour cycle. For most people, it runs slightly longer, averaging about 24.2 hours. This means your body naturally wants to push bedtime later and wake up later, every single day.
When you fly west, you're working with this tendency. Your day gets longer, which is what your clock already wants. When you fly east, you're fighting it. Your day gets shorter, and you have to fall asleep earlier than your body is ready for. That mismatch is what makes eastbound jet lag feel so much worse.
What the numbers look like
As a rough rule of thumb:
Westbound, your body adjusts at about 1.5 hours per day. Cross five time zones flying west and you'll be adjusted in roughly three to four days.
Eastbound, your body adjusts at about 1 hour per day. Cross the same five zones flying east and you're looking at five to six days.
For shifts over eight or nine hours, it gets more complicated. With a nine-hour eastbound shift (like London to Tokyo), your body might actually adjust "the long way round," delaying rather than advancing, because a 15-hour delay is sometimes easier than a 9-hour advance. This is why some very long eastbound trips don't feel quite as bad as you'd expect from the zone count.
Different directions, different symptoms
The two directions produce different symptoms, not just different severities.
Eastbound: the main problem is insomnia at night. You go to bed at 11pm local time, but your body thinks it's 4pm (or whenever). You lie awake for hours, finally sleep deeply in the early morning, then struggle to wake up. Mornings are brutal. You might feel a false burst of energy in the evening that tricks you into staying up late, which makes it worse.
Westbound: the main problem is early waking. You fall asleep fine at local bedtime (your body is ready, it thinks it's 3am), but you wake at 4 or 5am and can't get back to sleep because your body thinks it's mid-morning. Daytime functioning is usually better than eastbound because you're not fighting insomnia.
What to do about it
The strategies differ by direction because the clock-shifting mechanisms are different.
If you're heading east, morning light is your primary tool. Get outside between 6am and 10am at your destination. This pushes your clock forward (earlier). Avoid bright light in the evening. Take melatonin (0.5 to 1mg) at destination bedtime. If you can't get outside, portable light therapy glasses are a reasonable substitute. Force yourself to get up at a consistent local time even if you slept badly. Consistency matters more than total sleep hours during the adjustment period.
If you're heading west, evening and afternoon light is more useful. Stay outside or in bright environments until 7 or 8pm local time. This pushes your clock back (later). You generally don't need melatonin, but if you're waking very early (3 or 4am), it can help. Take it in the middle of the night when you wake, not at bedtime.
Does your chronotype matter?
Yes, though the effect is smaller than the direction itself. Morning people ("larks") find eastbound travel slightly easier because their clock already runs on the early side. Night owls find westbound easier for the opposite reason. But even a strong morning person will struggle with a nine-hour eastbound shift.
Age also matters. Older adults adjust more slowly in both directions, but the eastbound penalty is more pronounced. This may be related to lower melatonin production with age.
The practical takeaway
If you have a choice, schedule your trip so the harder direction (east) coincides with more recovery time. If you're flying east for a week-long holiday, arriving at the start of a weekend gives you two low-pressure days to adjust. If you're flying west, you can often hit the ground running.
And if you're doing a round trip, expect the outbound and return to feel different. The direction that felt easy on the way out will feel harder on the way back.